As of right now, summoning your Tesla Model S has only limited practical value.
For starters, the Model S is often plugged in charging, which means that it can't be summoned until it's physically unplugged (Tesla hopes to solve this issue with its snake-like automatic charger), but provided that your car isn't plugged in, what's a real-world situation in which the summon feature actually has a practical application?
DragTimes demonstrates one practical application of summon in this video - and no, they didn't time it:
"Demonstration of Tesla's latest software resale version 7.1 that includes Summon Feature. We demonstrate the Summon feature by having the car pick us up in the rain so we don't get wet!"
Backing your car out of the garage by itself seems a bit trivial, but avoiding a torrential downpour does seem to be a worthwhile application of this trick Tesla technology.