I think it's just to stay organized, maybe somebody would use a one of the sections to install games on and another to save all they're work files on...it's just what I think people would use them for.
Frankly it really doesn't matter. However lets say I got a virus iN C drive. I just reformat C drive. Don't have to get rid of D drive. I have two partions one for C drive which is XP. Its about 10 GBs the other is D drive which his for everything esle which is like 110 GBs