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.
There are gambling sites who are doing this, they are promoting their own token to increase its value once the sites gets popular, and this is a kind of gambling sites also that allows investment since their token will increase its value so if you hold you will gain, provided they have the success.
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.