Fix bugs. Implement new features. It's insanely powerful. Imagine a HUD that ships with a simple display of MPH and some other gauges. Then, sometime later (after you own the car even) Tesla is able to implement a fly-by-wire view that overlays navigation arrows on the roadway. They can just give you a software update and you instantly have the feature. It's why Tesla is referred to as a software company that builds cars. Over-the-air updates can give you NEW features. Remember Model S autopilot? That was an OVER-THE-AIR update to existing owners.