Specifically, to speak at other UU churches. (I enjoyed Ms. Kitty's post about doing pulpit supply.) More specifically, to UU churches who don't have a minister:
1) To teach about UUism. Take our church. We haven't had a minister in what, a couple of years? Three years. Wow. So we have members who are church members, but may not necessarily be UUs, because while they've been exposed to a lot of different speakers, they haven't heard a lot from the pulpit about Unitarian Universalism.
2) To show people why it's important to have the goal of getting a minister. Again, we have a lot of members who have seen a UU minister only rarely, if at all. Why should they get excited about getting a minister? What's so special about a UU minister anyway? And then, the Sunday happens where we get a dynamic UU minister in the pulpit. And everyone says, "OHHHH! Gollum want shiny UU preacher!"
3) To bring a little bit of the outside UU world into a church. It's very easy -- especially in small, minister-less congregations -- for church members to become myopic about UUism. Convinced that they are the only UU church in the whole world. And then a UU minister visits and "Look! A stranger! From a faraway land! But she's ... one of US!"
4) To say things that someone from within the congregation can't say. I travel around, doing pulpit supply as a lay speaker, and this is one of the aspects I really enjoy. If a member of Doe UU congregation gets up in the pulpit and preaches about forgiveness, well, some folks are going to find that a little pointed, whether it is or not. But if I do it ... hey, it's just a sermon. I don't know these people.
5) To get in touch with your inner circuit rider. Universalist circuit rider preachers are an important part of our heritage and by all accounts, helped grow Universalism "back then." Wouldn't it be great if the same thing happened now?
Preach on! Amen! Blessed Be!