Tasty Bacon! Crispy Bacon! I'd get it myself but I don't have THUMBS
WHERE IS IT!? Where's the BACON??
I'm guessing for branding to show "it's them", but also seeing facial expressions makes the experience more immersive?
If the Let's-Player is boring, then they don't get the audience. Half the fun of many such streams or videos is the reaction and commentary from the player and their peanut gallery. For some, this means that getting their face and reactions recorded is just as important as playing the game.
Personally, I find it distracting and don't enjoy it, but I can see how those that care about the player more so than the game would enjoy.