I very rarely watch Let's Plays for commentary- well, honestly, I rarely watch Let's Plays, period. But there are occasions.
1. Sometimes I hear that a game is great (especially that it's visually great, emotionally arresting, or has a fantastic storyline) but I don't own a system that can play it, for one reason or another. I might watch at least part of an LP to be able to experience something I normally wouldn't.
2. I may have played the game, but not experienced all of the endings. Some games require a lot of work to find all the alternate ending options; others just make it an uphill climb to get the "real" ending, period. (@$#% you, Painkiller.) Sometimes there are special endings that can only be experienced by doing very specific things that aren't easily accomplished by going back to a save file. I don't necessarily want to play another four to twelve hours of an eight hour game, mostly seeing and doing the same things over again, just to squeeze every last drop of narrative out of something I've paid for.
3. Sometimes a game is nothing I'm interested in, but I just want to see what all the fuss is about. (FNAF.)
4. On one occasion, I was told very specific things about a game's themes and storyline (Journey to the West), didn't have any way to play it, and wanted to be informed about it before I tried discussing it.
5. Sort of a subset of #1- sometimes a game is just no longer readily (legally) available. I love a lot of things about the era of the C64 and NES, but short of running an emulator, I can't experience a lot of them.
6. Very rarely, I just can't figure out how to progress- and FAQs don't always cut it, especially if it's a map issue or a "retrace your steps to this thing you never took note of" issue.
All that said, regarding commentary, with most LP-ers, less is more. And sometimes, more is a lot less (Shut up, kid!).