What is the point of preaching?

I've been thinking a lot about the point of preaching. I'm not doubting my calling or anything like that. I'm wondering how we could measure the effectiveness of preaching (like I said yesterday). But in order to measure something, it must be defined in some way.

For example, if preaching is supposed to make us understand God's word more, then we would need to come up with a scale of understanding and measure people before and after hearing a sermon. A successful sermon would show an aggregate increase in their understanding score.

But before we get to the scale, we need to start with the purpose. Why do we preach? Why is preaching a central facet of church life? Some people see preaching as an act of teaching, so it should be educational. Others see preaching as a prophetic word from God so it should be convicting (i.e. hell-fire-and-brimstone). Still others see preaching as an inspiring word so the church should be uplifted and encouraged. But others would argue that preaching is for the purpose of evangelism so the sermon should invite the lost into the kingdom of God.

In my experience, we often don't have a clear purpose in mind for preaching so we try to do all of the above and then we end up doing nothing well.