Not always. Several youtubers (particularly ones that don't stream) will play, then go back and add a commentary track.I suppose I should have said odd isnt a streamer, but I kinda lump them all together.
Edit: I know some streamers post to youtube and some youtubers occasionally livestream but they all seem to fall under the umbrella of entertaining you while they play.
RetconRaider does this a lot (cuts busywork and adds RPG story elements), and it can add a lot- I hope he does a battletech run on release- its his type of game. (Has long xcom2 and battle brothers runs, and covers a lot of indie and kickstarter project development, particularly for turn based tactics and rpgs)