- being able to quickly swap one pane to show a different file, then swap back to that same file again (or do it with the right pane)
- being able to horizontally split either vertical pane, ie you can tile the panes exactly how you like very fast.
:mouse=a
and now you can drag the split around
- showing differences between to versions you have open
:windo diffthis
highlights them - can't really do it at all with tmux
- Having the whole thing inside a screen session you can resume it from where you left off over ssh remotely later.
- Saving it as a vim session if you want to power the box down and resume from where you were, with precisely those splits and buffers open scrolled to what you see now.
- Backgrounding the whole thing to do something else quickly.
Yeah, there's a few, I doubt that list is exhaustive and doing things that way means some other trade offs. Whether these advantages are actually things you want is a different question that only you + experimentation can answer..?