I buy them because I'm too busy to write my own ... or something like that.
I run adventures like I'm using recipes in cooking: I start out with the written text, modifying and changing it as I go to suite my own taste. Every now and then I look at the written text and re-calibrate the process.
The last close to complete adventure I had written was five or six years ago. It had a consistent plot, cool villains, surprises which had the players lauding the villain's way of operation, a climactic fight in the end, and the villain's escape with a lot of story threads for the players to follow up.
The problem is that I spent an ungodly amount of time writing and re-writing the beast. At some time in the process, it turned from being cool to being a chore, and I realised that I modified my adventure at least as much as I did with bought ones. But while I tend to improvise my modifications with pre-written stuff, perhaps adding a sticky note, I changed the text of my own creation every so often and made sure that the whole text remained consistent.
I may repeat this ordeal, but only if I'm convinced that the idea is worth it.