You can be spontaneous with mobile ordering unless you suddenly decide you want a popular counter service spot, you want it right now, and you can't get a time slot until two hours from now. In those cases you have to either book your slot earlier in the day (I don't mean eat lunch at Woody's Lunchbox at 10:30, I mean get a 12:00 slot at Woody's the minute you walk in the gate that morning), or else you have to be willing to settle for whatever place is available last minute.
After you've got your mobile order time slot, the only difference is that you choose your food from the menu on your phone instead of standing in a queue looking up at the menu board in person.