Usually used when comparing things in a funny way.
One item is so vastly superior that it just fucking bitch slaps the competition.
Or that both items are equal and therefore the funniness is gone and the term becomes more literal.
That bitch and this bitch? No contest my man. This bitch is the shit.
Xbox 360 and PS3? No contest, your shit just RRoDed your ass and my PS3 YLoDed.
Mac or PC? No contest there man, I figured out how to get PC on my Mac. 'cept the GPU sucks.