Why are warm-ups considered beneficial
before hard labor or exercises?

A good warm-up will raise your body temperature, which is particularly helpful to your muscles.

Explanation: As your muscle temperature increases, oxygen becomes more available to your muscles, allowing them to contract and relax more easily – so you'll be able to perform more strenuous tasks with ease.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.