Spiralbound has good points, and I concur with them pretty well.
SpiralBound said:
1) Why do you create your own settings?
2) Why do you use your own homebrew settings rather than using a published one?
3) Am I still homebrewing if I'm borrowing elements from other settings?
4) If your homebrew setting is so vanilla that it's practically indistinguisable from hoardes of other such settings, then why bother creating it?
My answers.......
1) I've always loved to design, build, tweak, and generally 'create' things, even if only in my imagination. As a kid I played with LEGOs, and I sketched, and occasionally wrote down my ideas. Nowadays, living in Arizona (as opposed to my hometown in Michigan) kills my artistic spirit, so my only creative outlet is devising roleplaying game-related material. I try out different ideas and see where they lead me after a while. There are so many different ideas I want to try working on and see which ones turn out cool.
2) Originally, around 8 years ago IIRC, I devised my first D&D homebrew setting, Azeria, for 3 reasons, but the main reason was that I didn't want to be getting flak and problems as a newbie DM and D&D player. If I used a published setting, the players would inevitably know far, far more than I did about the setting, and I had already, in my brief experiences as a player, seen what kind of arguments and other problems cropped up when that happened in other folks' games. Some people just won't accept what the DM does with a published setting if the DM ignores (whether by choice or simply by not knowing) even one little piece of published background/story/rules involving that setting.
So, by concocting my own somewhat-vanilla 'everything but the kitchen sink' homebrew setting, I wouldn't be disadvantaged to the players when it came to world-knowledge and creative freedom. The players and I could all have fun and discover things through the course of a campaign, and not have stupid arguments about
this or
that obscure little published fact. We could use any cool or fun ideas we had without being restricted by published setting material, and I was free to allow or ban anything as necessary for the continuation of play.
3) Absolutely. Even if all you do is use the basic description of elves, dwarves, and such from the Player's Handbook, you're essentially using material from another setting. And besides, I tweak everything a bit, even if only in background. Hell, if you use typical fantasy elves and dwarves these days, you're pretty much borrowing material from J.R.R. Tolkien anyway, unless your elves are unusually and drastically different from the Norse alfar myths Tolkien drew from (or the svartalfar ones drawn on for Drow in D&D's Forgotten Realms).
4) The fun of creating it. The intellectual and artistic enjoyment of the creative exercise. Also, as others have said, you play it because of its differences from other vanilla settings, for the things you designed it to 'do right' in your own opinion, for your own preferred style of play. You play it for the neat little quirks or fantastic locations you built into it that are missing from all the other vanilla campaign settings you've read about. You play it because it's your baby, and no one knows it better than you, and no one's more familiar with it than you, and you have complete creative freedom to have fun with your own game setting. And maybe the rest of your gaming group loves it too; maybe they even worked with you to create it.