Even if you don't agree with my opinions or really think I'm kind of a jerk---check it out. I promise it will be to your long term benefit!
I wasn't going to post, but since the obvious retort doesn't appear to have been posted yet:
Want to know why the vinaya rules exist? Read the vinaya. If you don't want to read the entirety of the vinaya itself, read the next best thing. Ajahn Thanissaro's "Buddhist Monastic Code". I'll even post the link: http://www.accesstoinsight.org/lib/auth ... intro.html
Of course, the simplest answer is also the most obvious. To prevent clinging to food.