Which is why I said "largely", you answered and I accepted but I want each individuals answer because so many people are giving the same answer of "engaging" the audience. However this now leads me to a couple more problems.
"Their personality shines through on the cam much more so than without it". This I don't get. Exaggerated reactions and expressions don't convey personality, it just shows their exaggeration. If a person has a fun, easy going personality then their face isn't what shows that off, them being fun and easy going is. It could also be used for some hollow "personality" in that they act goofy expression wise because they want to be seen as fun and goofy and engaging but they might not be, they may be a bit boring when they are actually just commentating and so the expressions make up for the "lack" of a personality. I don't know if I'm getting this across well but to me a face cam showing someone being animated can compensate for the person not being genuinely entertaining through their personality.
"Their streams would suffer, in my opinion, without the face cams as I would be missing a ton of information". I don't see how, the information you'd be missing is "what face are they making right now?", that's about it.