Beyond the obvious answer (looking at two files at once, or a shell, or compilation buffer, or docs), I have a much simpler reason. My monitor is wide enough that if I have only one window, then all the text will be on the left end of the monitor, which means I'm constantly turning my head to work, leading to neck strain.

So even if I'm working on one thing, I do C-x 2 and use the right window, because then the beginning of the text is aligned to the center of my screen.

Yes, this is because I insist on running Emacs maximized.

C-x 3, I do the same :)