Allow me to explain this. Superman the movie and Superman II were originally one script. In this script, the turning back time device was used as the ending. When the massive script was broken into two pieces, it was decided to move the ending of the original script and make it the ending of the first film. Donner was shooting the pieces of II he had when he was fired, before a proper ending was written.
I actually already knew all this. It just seemed to me that given that the Donner Cut is basically an unfinished film, there was something that was written, but not filmed (sort of like the audition scenes) that would explain why he turned the world back at the end of II. As it is, there doesn't seem to be a point to it.
I'm not to sure that there ever was a solid reason. The world is bit beat up, Lois is depressed, and Jor-El died (which is funny, as he is already dead lol) but I agree that theres not really any strong motivation as is.
I much prefer the White house ending myself. That's a strong scene and has a real feel good quality to it.