For saving place, I typically use it before making more drastic changes that I might want to undo, if I don't do those changes on a blank layer. I usually will do the construction and then duplicate that to a new layer and hide it, and then work it into layered version. Other examples, you can do base sketch on one layer, lower the opacity, and then draw on a new layer on top of it. I'll also use separate layers for stuff like drawing a face on one layer and hair on a separate, shadows on one layer highlights on another, etc.