Your legs have the largest muscles in your body.
Training your legs hard has benefits that go beyond shaping them
and making them strong.
In fact if you want biggger arms, you should probably train legs!!!
This is due to the amount of natural growth hormone released by training legs.
"Here's why: the largest muscles in your body are found in your lower body -- your glutes and hamstrings. By strength training these muscles, your body generates an enormous quantity of growth hormone. Once this growth hormone is circulating in your bloodstream, it actually enhances the growth of muscles throughout your body. In this way, working out your lower body actually helps you build more muscle mass in your upper body and gives you a much more balanced, healthy, fit look. "
http://www.naturalnews.com/001506.html
http://www.bodybuilding.com/fun/sclark14.htm
This is the same hormone that keeps you:
1) Looking young.
2) Gives you a strong immune system
3) Decreased body fat percentage
4) More toned look.
http://en.wikipedia.org/wiki/Growth_hormone
So training your legs is very beneficial!