Sometimes they also used their own token as bet regard of they want to promote their project to people so they can reach more investors to invest in their project.
However, TBH, I am not fan of this kind of site as we know how unpredictable the market is, anything could happen and if such token is not listed in different big exchange, liquidity might be our problem in the long run.
I don't use their token because I prefer to use the other coin besides of using bitcoin. I think sometimes we are hard to find on what market was the token listed, and maybe we only need to send some bitcoin or ethereum in their site so we can convert it with their token. I feel that to gamble with the altcoin will not use too much money off me compare to use bitcoin. I also think about the price for the token at the market, and I don't like buying a high price for the token.