It
can boost sales, but it doesn't always.
This is an interesting video on the subject from a few months ago by TotalBiscuit, and the game he's specifically focusing on had a video with over half a million views by JackSepticEye (?), but the dev claims that they only got about 20 sales off the back of that coverage.
Check it out. That figure is mentioned about six minutes in:
https://youtu.be/Q4F-zdpFb9I
The video by Jacksepticeye most likely got half a million views because of him and not the game. The game in question also looks very disorienting and annoying, I'm not surprised that it only made 20 additional sales. I'm sure the developer spend a lot of time and love to create it, so I feel a bit bad bashing it, but the footage made me quite dizzy after a while.
Subscribers to these YouTubers probably automatically watch every single new video they do, so that high view numbers means very little in the context of the specific game.
I guess the type of game that works with this type of audience is "that looks hilarious when (screaming hyperactive zany facecam dude) does it, I will have similar strong reactions if I buy this!", like Flappy Bird or Happy Wheels. Now you can scream FUCKKKKK just like EpicReactionBro97!
Someone brought up Skate 3 played by PewDiePie, and I would guess that most people bought that game for the moments when the game flings your ragdoll body around rather than the in-depth skating mechanics.