I find that these are a great way to practice three things:
1) Working on the essentials FIRST - If you focus on some special effect or piece of art rather than the meat of the game, it'll suffer for it.
2) Scoping - If you dream too big, it'll bite ya.
3) Thinking Smart - If you design the game to be something that can be made easily, you'll do a lot better.
These to me are some of the most important qualities of a good game designer, even higher than designing good mechanics and such. If you can't handle these, none of those beautiful ideas will ever exist.