A lot of the larger casino sites use pop ups and that is mainly becuase pop ups (despite how much we hate those buggers) get a lot more click throughs than any other form of advertising.
Some sites will use cookies (which are 100% harmless and are easy to deny or delete) to track how many times they’ve showed you the same popup, so that they can show you different popups each time you come to their website.
The affiliate program would credit the person that sent the person and made them sign up in the same visit 