Because it can carry, potentially, outsize loads that the C17 can't at 5 times the speed of a ship, in a straight line without the need for a port or airstrip...ie almost to the logistically hard 'last mile'.
Think of it more as a very fast naval freighter and it makes more sense.
Provided that it is going downwind and there are no mountain ranges for it to cross... oh and provided someone has had the forethought to preposition tethering devices, ground handling etc at the destination oh and fuel for the return; sounds like it needs an airstrip to me.