1: Because if it's done on the timer the car gets the exact amount of run-down as calculated by the timer. Just sat there waiting *might* not be long enough dependant on how you drive, the timer negates this requirement for the user to think about how long it needs.

2: It's not just a timer..as I explained above. Some multi-functioning cool shit going on.

3: It's funny when un-educated people say you've left your car running just as it shuts-off