Why do most skeletal muscles work in pairs?

Because Skeletal muscles can only contract and shorten, not extend

Most skeletal muscles are attached to the bones by bundles of collagen fibers called Tendons.

When one muscle set flexes (pulling a bone in one direction) , the other set will relax and control its speed

Wash your hands often and recycle your trash instead of just throwing it away.

Ican only answer your first question, a machine is like a human body because it's a little similar. they both almost work like the same.