Nowadays, it's more common that casinos exist a while before their team figure out that they can get some good exposure at Bitcointalk. It can be from one of their team members or introduction from third party marketing people.
You can notice that if a flow is like a casino launches their announcement thread here and shortly like hours or some days later, they launch a signature campaign, I am sure that there is a signature campaign manager who told the casino team to launch their ANN thread in the forum.
They are on the neutral ground so they can get a good rate of their reputation if they'll be able to build it. Having their own forum is good but it looks bias since they owned it, but here, the competition is big with the existence of the new and the old and established casinos, so people who wants to read some real feedback will come here. And besides, bitcointalk is the most popular crypto forum in the space, so it's the best venue in terms of advertising.
Maybe not reputation but rather they want to create a platform so they can track the activities of their gamblers and make their casino known. But it turns out they can't maximize their exposure with their platform since generally it only dedicated to their costumers so provably that there's a lot of gamblers would not visit their forum since they maybe loyal to this forum or other casino.
That's why maybe a lot of casino choose to come and do their marketing attempts in this forum since they know that there's a lot of people visiting bitcointalk and its good for their casino to get a global exposure since from that they provably have a huge chance to became a successful casino especially if they have a lot of options brought to market well their casino on more other platforms that can possibly give them huge boost of their exposure. Bitcointalk is really the most popular bitcoin platform that's why its expected that more businesses will came here and introduce their services to a lot of people who visit this forum on regular basis.
That's probably the main purpose, to keep track of the activities. But if I'm a gambler, i would not focus on one forum only to check some feedback especially from a casino's forum as it can be moderated, admin could make some shady stuff deleting post that are against them. That's why I said we are in the neutral ground since bitcointalk does not promote their own product, all they do is just to implment the forum rules to ensure transparency among members.