Merchandise, such as t-shirt, is important if the casino has a strong community since it acts as an identity to make its users proud.
I'm not sure if gamblers have a small community to show off their wealth in public unless they are just servants of a great gambler or just bodyguard. If you are an e-sports gambler, then they have a community and support in each of those matches.
Perhaps, in one city which has a lot of people who gamble in the casino, they make a small community to share anything they want. So if they can have the merchandise from the big casino place, that will make them proud because not all gamblers can enter to that place. Or they can wear the merchandise when they are in the public area so that they can help the promotions from the casino too. The casino itself will take the benefits from that because many people will curious with the place, and perhaps, that can make them visit that casino.