Apple created the IPOD in order to reinvigorate their slacking sales. They created a device which would allow them to compete for the same psychographic that Voodoo, Alienware, and others are competing for. You cannot play video games on Mac OS, but you can certainly make a cool product that early adopters will latch onto.
The original IPOD customers are early adopters of technology, and these people tend to be influential when it comes to technology. These customers started using the IPOD and they became evangelists for the brand. Apple then managed to crack the mainstream consumer market with their IPOD as a result of all this influence. This is really the ultimate business model, get the early adopters talking about it and eventually Madonna will be seen jogging sporting her IPOD and the mainstream will bite. In the case of the IPOD this all happened unexpectedly fast.