I use one sometimes to clean up a wood barrel's hole. Not every time though, as sometimes the bit does fine. My reamer is old, junky, but it still cuts.
Anyways, no one has mentioned what I consider the most valuable reason to use a reamer. IF I was one to make a large quantity of calls for customers there is NO DOUBT I would use a reamer. The reason? Replacement parts. Say someone loses an insert out of one of my calls and orders a new one (either friction fit or O-ring). If I reamed that hole, I know exactly what insert to make that will fit perfectly in that bore. On the other hand, if you do not ream the hole you almost have to either hope for the best when you send off the replacement or just ask the customer for the barrel to make sure it fits.
-Cody