The start button (or app, or whatever) absolutely does something, and to say otherwise leads me to think you need to dive in deeper to how they work.

The button closes the contactors, activating the high voltage battery pack. To do otherwise is a massive safety risk. It also verifies the key (to prevent theft, and required by law) and on some models launches the parts of software needed for driving.

I'm not familiar with Tesla's design, but it should be easy enough to set the code to run this process whenever the door closes. Whether that's what people want is a different question entirely.