It's really handy to have a preview that's at the resolution of target textures when painting textures, I make a detail change in photoshop, save a copy in iterations, and reload textures in render settings. It's instant feedback when I make an element movement or addition like I'm doing right now with these Black Metal Corpse Paint WIP type makeups I'm working on for La Femme... Usually the actual realtime preview is good enough to work by, rendering it through preview renderer is just if I want to see it anti aliased. Preview render takes about 2 seconds, useful for toons and animation also; probably it's biggest advantage.