And it gives more context to law enforcement when they can see faces, body language and hear words rather than hearsay.
Assuming this is the reason, it seems much less trivial to install more/better cameras in a store than to coordinate this effort.
I think you'd be surprised how hard it is to cover 100% of a store floor. As another commenter pointed out, even if you can angle matters. Getting a shot of the top of someone's head is a let less helpful than getting a good angle of their face.