My guess is - I don't know, that it probably isn't seen as an end in itself - it's probably just skills practice, right?
I guess it fulfills the same niche as dryslope snowboarding, treadmills (or exercise bikes) at the gym, or indoor climbing walls. It's not supposed to be a replacement for the real thing - just somewhere to practice and keep your chops up mid-week, or when you can't bust out to the real thing.
No, it doesn't massively appeal to me either, but then I live reasonably close to some halfway decent trails...