Well, lower quality cuts, at the very least.
If they're actually selling bad meat, then that would be a very bad thing.
It's probably not bad meat, but it's probably closer to being bad. If the edible shelf life of the meat is, say, a week, they could easily take 5 day old meat, marinate it, and put it back on. That serves several purposes:
A - It makes the meat shiney again.
B - People will be more likely to buy marinated meat with a short shelf life, as they will assume it's the marinade that will go bad, not the meat.
C - If the marinade is acidic or salty, it could extend the life of the meat somewhat.
D - Possibly, the meat's flavor/smell deteriorates before it becomes dangerous to eat. A marindate could obscure this. (I just threw out some chicken breasts today because one of them smelled, very faintly, like eggs. The faintness of the smell meant that it was probably entirely safe to eat once cooked, but I wasn't willing to risk it. If it was marinated, I would not have noticed. And from the supermarket's POV, as long as they are not breaking any laws, it's preferrable I get food poisoning (which could not be necessarily tracable to the meat I bought from them), than bad smelling but safe meat, which is tracable to them and will discourage me from buying more meat there.