If you book on the Avis website direct, you can walk up to a faster lane at SFO on the third floor of the rental centre and avoid queueing with the people who ordered via a partner website. At busy times this is useful, e.g. when a BA flight lands (BA partner with Avis so the queue is at those times full of several hundred Brits queueing for their flight+car package motors).
If you have Avis Preferred Plus you can go down one flight of stairs after arriving at the rental centre and go straight to a bay with your car waiting in it.