What is the purpose of single, detached bones?

It is a complete sentence.

It could be used to handle difficult areas that are not connected, sometimes I add butt or side hip bone(s) that is not part of a chain but is used to tuck in odd deforms.

It could be a pole target, it could be used as a track to constraint, oftentimes eyes have a floating bone a couple feet out that the eye bone points to.

Other stuff.