If it were that simple this article would come to an albeit brief conclusion, but there is a little more to it than that.
First, the real reason why it is critical to your diet is because it is the building block of all muscle tissue in your body. You are building your muscle tissue by eating animal muscle tissue because that is essentially what it is.
Protein gives us branched-chain-amino-acids that our body cannot produce and they are essential to ensure that we can rebuild our muscle tissue after we over-work it. These BCAA's are only found it animal-based protein foods and there are nine of twenty-two that our body cannot produce which makes it that much more important to get quality sources of protein into our daily food intake.