Unneeded, thanks to disable it. 
Yes it’s just one step of a project to improve external links editing UI. Maybe the initial post of this topic didn’t recall it because the project had already been announced on the community forums beforehand. Note that plans are slightly changing as new issues are found. See also tickets @yyoung self-assigned.
It will be mainly useful to newcomers that do not understand why a different URL than the one they entered has been added. It will also help more advanced editors with noticing bugged URL cleanups.
Ultimately, it should allow to bypass the clean-up. Using this option should be noted in the edit data too, and can simplify reporting broken cleanup. However, this is future work that requires deeper changes.
External websites evolve all the time which fixing cleanup takes at least two weeks. It happens very often as we maintain a large number of cleanups. So it seems useful to bring more control to editors over links being entered.
Great, because nobody suggested this. The pop-up would be for editing links not adding new ones.