Not much. A horizontal split (C-x 2) can be quite useful to keep the top of a file in view while editing around at the bottom of a file. A vertical split (C-x 3) is useful when comparing files side by side. But that's about as far as I go, beyond that it just becomes annoying, as popups like from magit will show up in random subwindows, often rendering them unusable, due to that subwindow not being big enough.