Why is stress considered important in a shaft?

A certain level of stress is very important to get work done successfully, if we go below that certain level of stress, it produce silly, ignoring & irresponsible attitude which cause less and inefficient work. if we go above that certain level of stress, it became tention which also result in low efficiency...

I am a structural engineer, so in a shaft that I use for shear stability I like stress, preferably prestress, but I dont think that is the real question.