There's a handful of reasons to keep even beef in the equation, albeit at way, way lower levels than we produce it now.

One is co-production of other materials -- wool is another desirable material.

Another is optimizing land use. Some land isn't suitable for row crop agriculture, but can sustain grazing. There's also land we're irrigating for row crop agriculture right now from aquifers we really shouldn't be; converting it back to grassland and grazing on it would let us recapture some of the lost calories.

Then there's un-optimizing the food chain. From a food-per-acre standpoint we should be primarily subsisting on soy and whichever of corn, potatoes or rice grows best in the local soy & climate. Even if we are going all in on minimizing land usage, however, we don't want to produce exactly as much food as we need, because crops fail and famines are bad. You can aim to overproduce a little year to year and maintain a surplus of storage to cover the bad years, but you can't just cover the average bad year. You need to cover the worst years possible happening several years in a row, you need to be covering once-in-millennia bad luck. So you need to overproduce enough that you don't expect some of it to ever be consumed by humans.

So you need something to do with the extra corn you produce, besides feed it to people, that you can rapidly cut back on in case if crop failures to return food back to the supply. One option is ethanol, and another is livestock -- feed your surplus grains to livestock, and then eat them instead. If you have crop failures, you cull the herds, which frees up their feed stocks for human consumption, as well as adding the extra meat calories to the supply. (And this even happens somewhat automatically with market forces -- farmers are always arbitraging grain prices against meat prices, and a spike in grain prices will lead them to sell off livestock as they become less profitable, which will in turn depress meat prices further.)