PewDiePie is a well-known internet personality and the biggest YouTuber. Knowing that has over 50 million subscribers and billions of views every year. PewDiePie is also one of the most watched gamer on YouTube.
He is often plays indie games and free to play games suggested by his fans or some he finds himself. He has also made his own games, Pewdiepie: Legend of the Brofist is one example.
By playing Indie games, he promotes the games directly. Many developers request him to play their games. Whenever he plays a video game especially an Indie one, the sales of that game rises which is not only a good thing for the developers but this also introduces the viewers to a few newer games.
Being a very influential entity to the gaming industry, him reviewing a game and calling it bad (which he doesn’t) would cause the game to have a drop in sales or get bad publicity/reviews.
No doubt famous YouTubers effect sales, publicity and reviews of games. Especially for the Indie developers.