Have you ever been assigned to review another's code? Perhaps you had to add some features, or fix a bug. You've probably noticed that picking through another's code can be really difficult (if their code is not well written and well documented). I've found that it's always a good idea to have others look at your code to make sure they understand it. (Better to do it early than to do it once you've finished all your code!)

Also, by having others review your code, you can determine if you are documenting well enough by your peer's comments! You will also make good friends this way (or good enemies, depending on the readability of your code!). :)