Not only does it give you a spare, as Eagle noted, but the dual system is also considered more accurate. The vents on either side of the fuselage are connected in a "Y" configuration and help decrease static errors created by yaw in comparison to a system with only one port.
A little off base but i had an instructor once that told me if for some reason the static port failed and you needed the airspeed and alt you could break the face on the VSI. and it would act as a static port. Anyone else know this to be a fact.
This will work as the static air will come from cockpit. The reasoning behind breaking the face of the VSI as apposed to another static instrument is mainly because of its lower cost and that it is not required for VFR or IFR flight.
In the Piper Arrow/Archer, it's not THAT much of a worry, since there is an Alternate Static Source under the Pilot's yoke that will fix the problem (to a point). However smashing the VSI will help, although there will be instrument error. Kinda a "last dtich" effort.