Because I like to find all sites in my lands.
There's only one non-unique H3 site. It's uncommon, and it's total crap (it just lets you recruit priests). However, finding this site can mean that I've now found 4 sites in the province and do not need to waste time or gems looking for paths that I have not searched yet.
Sorry, I did forget one other situation which calls for preaching: removing damaging enemy dominion (LA Ermor/LA Rlyeh/MA CTis).
But my main point was that a prophet that can reanimate is much better than a preacher. I would never use an undead or demon prophet for site searching, unless there was a hint that there was a holy site in the province (holy fire/less unrest/+50 gold), and I has no other priest available who could search it.