Yeah, I agree with Slothy. Until Dawn is an interesting example to bring up, because I saw an LP of the initial section of Until Dawn and thought it looked great. I stopped watching it and bought the game itself because I wanted to experience it for myself. I was right, as I bloody loved it. I watched the LP after I had beaten the game myself too because I wanted to see what the LPer did.

Most games developers, particularly with smaller indie games, show a spike in sales when the big YouTubers like PewDiePie, Markiplier, Jacksepticeye et al play their games.