Two different reasons for this, I'll go with my first explanation.
When dealing with someone new to the idea of licking an asshole, I want to tell her while she's got my cock in her mouth. Watch her nose crinkle at the sound of it, look up at me to see if I'm serious and then realize I am before she works her way to the back reluctantly.
Otherwise, I go the other way around.
Tall or short?