Yes, there is. Its the same reasons new and growing developers put games on Itchio and GameJolt. They can get feed back on what people think of the game, can get and see if there are bugs, and its FREE exposure for the game and the game developer.
If you don't wanna watch, then so be it. But many others do. When a game is being played and uploaded by alot of people, the more people see it that might have not seen it. Plus it helps people find creators they might like, who might show them even MORE games.
So look at it this way, it not just uploading a gameplay to itchio.. its then featuring that gameplay to others who follow you or watch your content from other games you have played. Hence the reason why you see it on every game. It content creators and devs working to promote and grow together. It doesn't just work one way.