Hi Glenn,
Usually on the side walls close to the speakers to minimize reflections however still keeping some energy in the room. Some say it also helps with imaging.
The benefits are basically what is written above. Recording a source, such as Piano, Drums or really anything else close to a diffuse field will sound a little more natural (for the lack of a better term), and not so dead (again overused term) as in entirelly absorptive spaces.
I hate to be so blunt, but just try recording in diffent places in a room and find the perfect spot for that instrument/song/style and etc..
Hope it helps
Gil