I can only think that its a distance thing; say your office is along ways away from the truck and you wanted to remote start it a few minutes before leaving work, with cellular communications distance becomes a non issue for the range it would work, other than that I'm not sure either.
For me if the app had more than just remote start; like lock/unlock, vehicle status such as, trip A/B info, miles to empty, vehicle location, etc would be nice to be able to do from a smart phone, though I would then have something else to worry about as far as hackers trying to fine a way into my truck... (just thinking out loud).