I think urine marking serves a lot of purposes. It helps all involved to know where they've been, who they trust, what they can eat, what they shouldn't eat, that they're full of testosterone and would make a good mate, or that they're a sire-seeking female. Lots more reasons, too, I'm sure.
http://www.ratbehavior.org/^that site discusses a lot of behaviors as well as urine-marking.