It depends what you are grazing and what your next intended use is for the parcel of land. Some animal manures need an extended period to break down properly and if you are sewing more sensitive crops into that area in the next cycle you’ll want to remove more recent deposits or you’ll end up with patchy germination.
Traditionally the cycles wouldn’t be that rapid so it wouldn’t be an issue and you’d rotate something else in between.
Of course that’s only one set of reasoning, people may also just want to use the manure elsewhere.
