Here's a list of the advantages (some apply more to female owners than to male):

we will do things their girlfriends/boyfriends wont - swallow, anal, cunnilingus, etc
we never say no to sex
we take care about our appearance
we always wear sexy undies
we are obedient
we never want sexual pleasure, so they dont have to worry about us
we will do their housework
we dont need them to buy us presents to keep us happy