I, personally, show little to no interest in a simple playthrough with a voice over of the player, actually, the voice comments annoy me (I would rather prefer wrotten ones), I think that a videogame is something you should experience yourself on your own, and while check a video won’t hurt, if you do this before you even play the first time this will surely hurt your first-time experience.
I usually only check satire LPs, those humouristic ones that can make fun of several aspects of the game and make jokes, and I limit them to those games I have already played, that is the only way that this kind of experience feels fun to me, I have no interest in watching a simple video when I can’t enjoy the game’s contents myself.
Harmful? Yes and No, it depends on the genre and how the video is made, VNs and some video game genres may have not too much gameplay to show actually, so once the user in question sees all what is of interest through someone else, they will have no interest in playing themselves, which will hurt sales.
On the other hand, other genres may help sales, like games that rely mainly on multiplayer or those with a fun or quirky gameplay that is worth experiencing at least once on your own.
The answer to this… it’s as simple as it depends on the factors in question for the game, we can’t generalize this, you could say every game is a world on their own.