If you go to a live event you don't just want to listen to music, you want to witness a show, right? You want the people on stage to be rock stars for the night. And you want to be swept away on a flood of shared adrenalin.
It's like religion. Why bother going to church when you can pray and read your scriptures at home? The reason you go is to achieve "togetherness": to unite with a bunch of like-minded people and listen to a preacher whip you up into a spiritual frenzy.
In church it's the preacher's task to orchestrate events, to summon the spirit and channel it to the congregation to make it shiver and shake and sometimes scream and shout. Rock 'n' roll bands have a similar responsibility to guide the vibes when they play live and, of course, a little charisma can go a long way. Otherwise we might as well stay at home and stick on a CD.