Collectibility. Real, factory shrink wrapped 1st Gen iPhones are up for thousands on eBay. Will they get that much? Probably not. 3Gs are in that 'throw in the trash' territory between being useful and $100 cause it's 'rare'.
Sucks that it's so hard to find a genuine sealed iPhone that's not purchased from a carrier or Apple themselves.
I purchased an iPhone 5 the seller claimed to be brand new and unopened. What I received was an iPhone that had been opened, updated, put back in the box and sealed with cheap cellophane.
The seller had 100% feedback so either I got jipped or a lot of naive people are buying and not aware of what they're getting.