They are a renewable resource, and utilize another renewable resource, plants, to produce these products and services. In addition, the manure produced by the animals helps improve soil fertility and, thus, aids the plants. While giving these interviews slamming domesticated farm animals, PeTA's Newkirk is pictured cradling a domestic cat. She is often photographed hugging a chicken. If that cat and chicken and all the other domesticated animals had a vote, they'd most certainly vote against PeTA setting policy since, if PeTA wins, all domesticated animals will be history.
Animals are a more important source of protein than they are of calories.
It is true that it is more efficient for humans to eat plant products directly rather than to allow animals to convert them to human food. At best, animals only produce one pound or less of human food for each three pounds of plants eaten. However, this inefficiency only applies to those plants and plant products that the human can utilize. Only about one-third of the land area of the world is classified as agricultural. Thus, roughly two-thirds of the land area of the world is not suited for any sort of agricultural use because it is covered by cities, mountains, deserts, swamps, snow, etc.