I think I can help clear up a little of the stigma around Let's Plays here. The main aspect people enjoy from Let's Plays is the personality of the commentator. The commentator should provide a huge contribution to the game and be able to connect with their audience. An audience for Let's Plays often wants to see:
1, How a person would react to a funny, ridiculous, confusing, or emotional scene.
2. How good of a gamer they are, and if they would fail the events of the game and humorously rage at the camera, or ace it like a pro.
3. The thought process going through the player's mind as they play the game. How they connect pieces of the plot and relate to the characters.
I'll use Jacksepticeye as an example. You probably know him as the 'Green PewDiePie' if you frequent YouTube's gaming section a lot. Jack gives off the personality of a humble Irishman, but also an emotional person who connects with his audience. He openly displays his emotions. If he is playing an emotional game, he gives off that aura of sadness a player would feel. It's as if you're playing the game with him or right there in the room with him, because he talks into the camera. He talks to you.
That's why Let's Plays are popular. Or at least why people get popular doing them. They connect with their audience, give the reactions people want to see, and not only that, but they inspire people. A lot of people try to do Let's Plays after seeing other people do them. You'd be surprised how many people PewDiePie has inspired.
Yes, Let's Plays are discovered from the game, search engine-wise, but the people who subscribe to them and continue to watch them.. They watch them for the personalities. They share an experience with someone behind a camera. They find someone they can relate to. They find someone they can be inspired by.