Why do people buy commercial modules when early RPGs assumed the GM would make up the adventures?
Simply put, game mastering takes time and effort. Game masters who use multiple sources requires significant demands on their time, something that is increasingly challenged by the diversification of other forms of easy entertainment. I discussed this in two different articles: Worlds of Design: The Chain of Imagination and World of Design: The Lost Art of Making Things Up
But it’s also certainly because adventures make game companies money. In many ways, making a game world out of whole cloth can be daunting to new gamers. It's just easier (and more lucrative) to buy adventures set in an established game world. This has the added bonus of causing a lot more commonality among the customer base (who can share tips and tricks with each other on how to play an adventure), and also happens to make those same game masters repeat customers as their players advance in level.
It wasn’t always like this.
But that all changed over time. D&D became more solidified as the rules went from Original D&D to Basic/Advanced, to just one version. Along with the codification of rules came established adventures, many of them now legendary in gamers’ experience who played through them (e.g., Expedition to the Barrier Peaks, Ravenloft).
Of course, not all adventures were fully fleshed out either. Some had large gaps (both in the maps and text) where game masters were meant to customize to their liking, or roll randomly to determine what came next so players wouldn’t be able to metagame the adventure. Over time, this became much less common, to the point now that we get completely mini settings. For an example of how much has changed, see Beth’s review of Quests from the Infinite Staircase, which takes sandbox-style adventures from Basic and Advanced D&D and fleshes them out in detail.
This trend is exemplified by Paizo, how pioneered the art of the Adventure Path. D&D’s level system ensures games take a lot of time and effort for player characters to level, which requires a lot of adventures strung together. A GM in the old days had to buy different modules and justify stitching their plots together, but with an Adventure Path the entire throughline seamlessly integrates from end to end, from the very first to the very last (usually 20th but not always) level. It's a lucrative model, as it requires significant investment from customers not just for one adventure, but for several.
And finally, as you get more experienced, adventure writing becomes a lot easier. There’s nothing like playing a terrible adventure to motivate you to write your own. I doubt that there are many veteran GMs who have never used a commercial adventure module – I certainly have used them, for convenience (lack of time) or when one was especially useful or even famous (e.g. Against the Giants). I haven’t bought one for a long time, because I already have so many, and because there are so many free ones available. But it appears from Wizard’s catalog, and from the publications of many other publishers, that lots of people buy them.
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Why Bother?
Of course, it’s much easier to use a module than to make up your own adventures. But there’s more to it than that.Simply put, game mastering takes time and effort. Game masters who use multiple sources requires significant demands on their time, something that is increasingly challenged by the diversification of other forms of easy entertainment. I discussed this in two different articles: Worlds of Design: The Chain of Imagination and World of Design: The Lost Art of Making Things Up
But it’s also certainly because adventures make game companies money. In many ways, making a game world out of whole cloth can be daunting to new gamers. It's just easier (and more lucrative) to buy adventures set in an established game world. This has the added bonus of causing a lot more commonality among the customer base (who can share tips and tricks with each other on how to play an adventure), and also happens to make those same game masters repeat customers as their players advance in level.
It wasn’t always like this.
The Hoi Poloi
In the early days of Dungeons & Dragons, lack of a single campaign setting (we had both Greyhawk and Blackmoor), ever changing rules and editions, and the general inability to share them (no Internet back then!) meant games were messy affairs. Game masters made things up as they went along, customized rules as they saw fit, and largely played what could only be interpreted as a variant of D&D. And for some time, this wasn’t just the norm, it was encouraged by then parent company TSR, who wasn’t in the business of publishing adventures.But that all changed over time. D&D became more solidified as the rules went from Original D&D to Basic/Advanced, to just one version. Along with the codification of rules came established adventures, many of them now legendary in gamers’ experience who played through them (e.g., Expedition to the Barrier Peaks, Ravenloft).
Of course, not all adventures were fully fleshed out either. Some had large gaps (both in the maps and text) where game masters were meant to customize to their liking, or roll randomly to determine what came next so players wouldn’t be able to metagame the adventure. Over time, this became much less common, to the point now that we get completely mini settings. For an example of how much has changed, see Beth’s review of Quests from the Infinite Staircase, which takes sandbox-style adventures from Basic and Advanced D&D and fleshes them out in detail.
The Art of the Module
There’s also something to be said for the art of adventure creation. That is, there are definitely some adventures that are better than others, and those who figure out the magical mix are more likely to be bought by game masters who appreciate the effort. Or to put it another way, people who create published modules will, on average, likely be better at adventure writing than a novice, so you might choose to buy a few to learn from the best.This trend is exemplified by Paizo, how pioneered the art of the Adventure Path. D&D’s level system ensures games take a lot of time and effort for player characters to level, which requires a lot of adventures strung together. A GM in the old days had to buy different modules and justify stitching their plots together, but with an Adventure Path the entire throughline seamlessly integrates from end to end, from the very first to the very last (usually 20th but not always) level. It's a lucrative model, as it requires significant investment from customers not just for one adventure, but for several.
A Question of Experience
Whether or not you buy published adventures likely pivots on several factors: your prep time, your players’ interest in a campaign setting, and your experience. Game mastering is a significant investment, so if you don’t have the time, published adventures are the way to go. Your players might be deeply committed to a setting (like Greyhawk) and thus be only interested in playing in published adventures in that campaign world; conversely, they may like your homebrew so much they could be turned off playing anywhere else.And finally, as you get more experienced, adventure writing becomes a lot easier. There’s nothing like playing a terrible adventure to motivate you to write your own. I doubt that there are many veteran GMs who have never used a commercial adventure module – I certainly have used them, for convenience (lack of time) or when one was especially useful or even famous (e.g. Against the Giants). I haven’t bought one for a long time, because I already have so many, and because there are so many free ones available. But it appears from Wizard’s catalog, and from the publications of many other publishers, that lots of people buy them.
Your Turn: Take the poll and let us know!
? I can usually edit a module in a few hours and then it's good for the group play. I guess if your focus is more Story® than rpg, I could imagine it taking more time to add all the literary elements associated with storytelling 
