Ease of use
I personally find it most useful to be able to comparison shop when I'm out in meatspace, shopping. I can browse in Virgin/HMV and then work out if it's worth waiting for a play.com or amazon delivery over immediate gratification.
But but those sites can be a pig to search even on a smartphone.
However I can definitely see the appeal of being able to do things like buy cinema tickets using a specifically designed mobile portal. Beating the queues by ordering tickets in the pub would be great.
There's also lots of possibility there of using the security provided by a SIM card to authenticate purchases to a pre-associated credit card account without having to go through the laborious details entering which would kill the mobile experience.