Example: making overdubbed audio from one language match a video that was spoken by actors in a different language.
Also for making parody/cover videos where an amateur vocalist is trying to match a commercial track and misses by a little bit, but that was their best take so you gotta keep it.
Or to help an amateur rapper get all their random verbiage in before the chorus starts…
As @Austin comments, it may be one of those options.
My case is as follows:
I have a TV series that was dubbed into Spanish at the time (mono audio).
Then there was a remastered release in HD and 5.1 channel audio, but there is no Spanish dub.
With Reaper, I can synchronize the Spanish dubbed audio track with the HD remastered video.
I can also replace the center track where the original voices are with the dub track, keeping the rest of the tracks for music and effects.