Create their own forum won't get much attention Imho. That's only make people joined on their campaign but there is no attracted people will come. Weekly contest is just regular way to attract people to come and play, i don't see it will attract immediately since it's just a normal thing on gambling site
Create a forum will require resources to handle many things. Spam, scam, accusations and the forum is big enough and many gamblers are already here.
This forum is perfect for gambling companies to advertise and if they are companies that accept cryptocurrencies, the forum is more than perfect to begin with.
The main idea of having its own dedicated forum is to build their own community. To give a better place for the players to communicate with other players. Just take a look at Stake as a perfect example on how they manage their own forum and what the effects for the casino. This forum is perfect indeed but not all casinos gain much interest from the forum member as most of members are not even gamblers.