I think what they are trying to offer is the "one stop shop" kind of gambling site. Having a variety of casino games and sportsbook will most likely attract gamblers, and since that has been the trend already, we can assume that the kind of strategy is working.
Personally, I also played some casino games, but not a lot, it's really sportsbook that is my main focus when trying to have fun.
Before, I knew some popular sportsbook nitrogensports and cloudbet, they are mostly focus on sports betting but I think they start adding casino games eventually. So IMO, this is a way to remain competitive in the market.
Yeah one stop entertainment, with this way any user who plays other games beside the sports betting doesnt need to withdraw and deposit on other sites.
This also raises the casino winning chance because their user will be more active with bets on different games within the platform.
The other reason is, many casinos using 3d party gambling provider which offers a complete package for everything including slots, live table, poker, sportsbook, house games, marketing tools, payment system, etc.
The price difference between a complete package and a custom package is not that big.