Sometimes you do some things then undo and redo trying to get back to a certain point in time. In vim you can step forward and back through all this by doing "g-" or "g+" to travel backwards or forwards in time. Or you can say ":earlier 50s" to go back to where you were 50 seconds ago.
Most editors treat undo as a linear thing, you undo and you redo. But once you undo and then do some things, it starts looking more like a tree than a line. It's not frequent, but sometimes I just need to say "I know what I had was what I wanted 5 minutes ago."