If you find
Livestock are selective and will graze what they want and leave what they don’t want
OR
Weeds build up in pastures
Then,
A Short hard graze with High stock density allows a paddock to be grazed flat or near to flat so weeds are taken out along with the desirable pasture or rangeland plants.
This basically eliminates any Selective grazing and puts the control back in the hands of the manager instead of leaving it in the tastebuds of the grazing animals.
This is one part of a Short on, long off rotation. The long Rest can allow enough time for recovery from the grazing and particularly can allow the pasture to bounce back strongly as a weed beater.