And to keep the nose down the runway.
Opposite rudder is used to lose height quickly...
During a crosswind landing, there are basically two methods: "crab" and "slip". With a crab you float down sideways until just before the flare. Then you push the windward wing down and apply opposite rudder (the slip configuration). With a slip, you just slip down final. Either way, you want your nose pointing down the runway when your wheel hits the ground :-) (Yeah, I said wheel, because you typically land (in a tricycle gear aircraft anyway) on the one main gear wheel on the windward side ;-)
Not at all an uncommon maneuver - and Todd's right. You practice them (at least I do) when I can. My home airport has one runway (10-28) with prevailing southwesterly winds, so most of my landings are crosswind landings anyway.
