Why would you lick an ass? Well, why would you lick a vagina? For the pleasure, right? Licking an asshole is much more taboo than eating out a vagina, and there's an aspect of pleasure there (have you ever had your asshole licked?) that adds to the orgasm
I've gotten off on facial expressions alone before, it's the subtle things that normally don't transpire with average, vanilla sex. You get to see another aspect of the person, it's another notch in the trust factor, not anyone is going to let you lick their asshole.. having the green light is an added factor in intimacy, similar to having sex. It's just something else she'll let you do, something she wouldn't let someone else do.. that makes it kind of special. The animal in me says "take it!"..