Why?
Seriously, is it that difficult to work out that Facetime is Apple's first serious attempt at getting away from the carriers? Devices with the capability to make video calls are rapidly becoming the norm in consumer electronics. Almost all laptops have webcams and most desktops can have them added for just a few pounds. More smartphones are starting to offer front facing cameras as standard and that number will surely increase over the next few years. Both the PS3 and Xbox 360 have existing camera attachments and new add-ons coming out this year which will also include that functionality.
The problem is everything works on their own standard. Skype and other VoIP services are out there yet somehow it's not tying together (and video call quality can be... dodgy). With Facetime Apple's looking to not only present an open standard for this sort of thing going forward but also provide such a large pool of devices that use it it makes little sense for anyone to compete. All the existing providers of video calling services are being offered the incentive of having access to all those iPhone / iPod Touch / iPad users (as and when the devices get updated). And Apple themselves will more-than-likely integrate Facetime into iChat at some point (10.7?)
Yes, it's wi-fi only for the time being but that's just fine. 3G networks are already over-capacity in many areas and 4G is on the horizon. In a couple of years 4G networks will start full blown rollouts and provide enough bandwidth to support this properly. At that point I'm sure Facetime will be extended to support it (or, if you prefer, the restriction on Apple's devices will be lifted, not sure if that's in the spec for the standard or not). At that point Facetime has the potential to be positioned in such a way as to make voice minutes almost an afterthought for a lot of users.
THAT'S the angle Apple's looking at with Facetime, breaking carrier dominance over voice and finally turning them into basically one big data pipe and nothing else or, at the very least, radically reducing the reliance on their voice network. Why wouldn't you want that option on every device you have (at least, where it makes sense to have it)?