For music originally released on vinyl the running time is going to be less than can fit on a CD, by quite a lot unless it was a double album. Filling that extra space with a few live versions or tracks that didn't make it onto the original release could help sell the album a second time, though often they're only of curiosity value and end up being skipped after a few listens. It's not the extra tracks that mess up the recording quality but the choices made in making the master but that's a different discussion.
