Sermon question time?

I'm kinda curious why there are sermons at all.I mean, I can see why sometimes you'd need someone to preach at you. But where did we get the idea that every week some guy should be forced to come up with some new variant on "Jesus, yay!" or "Sin, boo!" and the congregation be forced to listen to it? It's not always going to add value, is it? Couldn't they just read another chapter of the Bible and have another coupla hymns? But no, the poor old pastor has to think up another feeble metaphor or anecdote to illustrate to his congregation a principle that every single one of them already believes.