I type "A B C D".
Then I undo thrice to get "A".
Then I type to get "A E".
And now I realise that what I actually want is "A E B D".
My edits have created a branch is history. And I need to switch to the old branch and copy stuff twice (first "B" then "D"). That is why a tree is important.
I didn't heavily use undo trees so I don't remember how neovim/emacs displays it. I think I get it enough that if I try it out I'll be able to understand what you mean
