I've used it to merge outside the modeler once ( https://forum.mendix.com/link/questions/87599 )
And I've used it occasionally for renaming the project, as renaming the project in the cloud portal is not enough ( https://forum.mendix.com/link/questions/4498)
We have used svn in the past to roll back our project after an upgrade. I'm not sure if reverse merge changes would work now when changing mendix versions (haven't had to do that yet), but this forum post was very helpful.
In addition to the things already mentioned, I use the revision graph functionality quite a lot as well, mainly to get a quick graphical view on commits over all branch lines.