They control all matter of noxious weeds and grasses. During the winter dry season the sheep can survive on tree prunings. This past dry season was especially long and severe in Hawaii. The answer was to prune and top the citrus and avocado trees a little every day, as well as thin out the banana clumps. These trees produce a large amount of biomass which is a major disposal problem.
Many people haul it to the landfill, burn or shred it.
We do not have to expend this kind of cost and energy. Just starting the chain saw caused the sheep to come running over to eat citrus or avocado leaves. You may ask what about all the limbs and branches left. Driving over the pile with a tractor or using a flail chopper on a tractor rapidly eliminates the piles bulk, the fungi and bacteria do the rest. The point of this is that instead of wasting all that biomass, along with the energy and time to remove it we are instead converting it to animal protein and returning organic matter to the soil for humification.