In the online game industry, there are developers who create the games and publishers who release them. Some providers develop and publish their own games, while others don't.
Big companies like Microgaming or Yggdrasil are both developers and publishers. They have their own game studios that create new slots, but they also publish games from other developers.
This is a good deal for both parties. Small developers might have a hard time getting their games to multiple casinos, but it is easy for big publishers. Having a powerhouse like Microgaming publish your game will make it easily available for everyone.