I asked myself that same question: why does an archived cache need FPs? But, after reading through the related thread, I have since changed my thoughts.
For me, the idea behind a FP was to reward not just the cache but also the owner for a job well done. When I ran out of FPs, I had a dilemma: do I remove a FP from an archived cache to free it up so I can award it to the new active find? Obviously, I won't consider doing that anymore but, instead, I will just get my butt out there and find more caches and award FPs as I get new ones.