Convenience. Renting from a rental shop involved ordering online using a bad UI, showing up at an office, waiting in line, getting told the car you'd requested isn't available, would you like this instead, would you like to buy more insurance, make sure to bring it back with a full tank, and you're paying for a full day, even if you just need groceries from costco.

Zipcar involved looking for the nearest location in the app (typically a few blocks away, if near SF), picking your car and timeframe, showing up, and driving away. Fuel is included, just use the included credit card. No human contact or upselling required, and you can return the car at 3am.

If you only need a car for a few hours, Zipcar is much more efficient than a traditional rental.