Well, when I go do grocery shopping I sometimes have a paper in one hand. If I had a device to flag (or scan) the items I'd prefer to do that with one hand instead of two. I'd also want as little user interaction as possible, and make sure the item is also processed in my 'inventory', as well as the date it goes bad somehow tagged and read (yeah, dream on, maybe some RFID reader in future). The same would be true when I'd walk with my grocery bag to my car and someone phones me. The fact I can simply walk around and quickly navigate with my Nokia E71 and its little keyboard and dpad provides an advantage in certain situations; purely because of its design. However, there is not one perfect product suitable for everything.

You can also imagine that the application simply behaves different (in some ways more lightweight but maybe also allowing something good different because of change) and that this is optimized for the use cases.