"A sermon is like last Thursday's dinner: you can't remember what it was, but it did the job at the time." I learned this and other interesting things in my Expositional Preaching class. Actually, I'm not really in the class per se, but the professor is letting me attend in case somebody drops out of the class.
Why preach? While preaching isn't the only important job in a church, it is what most people remember about visiting a church. When you say, "Honey, what did you think about that church we visited?" your spouse will think first of the sermon, then the worship and other ministries.
Why preach? Another reason is that it is cost-effective; you can influence more people at once for God's kingdom. Also, there are some things that can be said in the anonymity of a service that people may not accept when you say it to their face.
Why preach? Because, like it or not, good preaching is the daily food that feeds the congregation. They may not remember it next week, but it "fed" them. Bad preaching could be compared to mac-and-cheese or TV dinners, I suppose...
That is why preaching is important and should take up a large portion of a pastor's time. When I've prepared sermons in the past (I've only preached twice, although I've prepared several more sermons), I have spent no more than 15 hours on a 30-minute message. My professor suggests that anything less is doing a disservice to the congregation as well as the Word of God; he thinks good sermon preparation will take closer to 20 hours.
I'll be preparing two sermons this semester... I sure hope I get better (I'm confident I will) because of this class. even if I don't get course credit for it, I think the time will be worth it.
I did ask how important preaching is to the health of a congregation. In a large congregation, in the absense of a pastor, good preaching is critical to maintaining church health. In a small body, if there is a strong sense of community, good preaching will certainly help, but its absense will not kill thebody. A small congregation without a strong idenitity will disolve quickly without good preaching.