I think they're mainly useful if you're working in a team, or if you're being paid to make the game (and need to demonstrate to the employer that everything is progressing as planned), or if the game is particularly complex. If you're making a small game on your own, you can just keep a lot of stuff in your head.
Having said that, I do write an awful lot of small text files, describing all kinds of things about the game.
For example, if I'm making a strategy game, I'll plan out all the different units, all the different types of terrain, the combat mechanics (formula to calculate damage, etc), the movement system...
The difference is that it only has to make sense to me - noone else will ever be reading it.