Deposits are being taken by Porsche and Audi because automakers, and this includes Tesla, know the size of the market for EVs.  Demand profiles with deposits are important in changing relative production levels as the industry transitions to EVs.

A bit of trivia: Ford had 727,000 for its Model A when it was introduced in 1927.  Interest was so strong that over a 100,000 people went to the dealerships to see the car on its debut and in New York, Madison Square Garden was used to contain the crowds.