It keeps it so that you do not burn off the muscle that you have. When your body burns calories, it burns them indescrimanately (both muscle and fat). Having sufficient protein basically protects the muscle and makes it so that you are burning off the fat. I do not know the exact science as to why it does this (hopefully somebody else will know) but it is also why you should only do cardio for 30 mins a session when trying to build muscle. Anything over that, and you can guarentee that you are burning your muscle off as well as the fat.
Ever see a very muscular marathon runner
? Doubt it. On the other hand, look at the sprinters. Those guys have great physiques because they train a lot differently (usually using HIIT "High Intensity Interval Training" methods), which eliminates the fat and retains the muscle.
http://www.buildleanmuscle.com/interval-training.html