Sportsbooks and W9

What's the deal with asking a person to fill out a W9 at a brick and mortar sportsbook? At most casinos, you have to have a players card with all of your information already, especially to place a big bet, so I don't see the point in asking for the info (again). Beyond this, in a recent example I saw, I don't think it had to do with a CTR, since amounts were close but clearly not over the threshold. They say on the W9 form it's just for an entity/employer but is not reported. I've heard from others that this is just another way for you to confirm one's SSN, but again why would one need that confirmation if they aren't reporting? Just pay the line.