It is a bit more complicated, but one scenario is obvious: you can feed a cow 10,000 calories of corn to produce roughly 2,000 calories of beef. If you simply

It is a bit more complicated, but one scenario is obvious: you can feed a cow 10,000 calories of corn to produce roughly 2,000 calories of beef. If you simply eat the feed instead, you'd be eating calories that are 5 times more efficient. However, beef can also be produced on marginal lands (grazing) that aren't suited to crop production. In this way, livestock can be seen as an efficient land use (caveat environmental damage from grazing, etc.). However, this does not represent much of current livestock production.