Reduced need to dedicate public space to storing private vehicles, a more efficient fleet (since they're incentivizes to optimize for cost/mi), no time spent looking or paying for parking, no need to maintain or fuel a vehicle, reduced risk of impaired driving, more experienced/capable drivers, freedom to switch between transit modes (cycle/walk somewhere but call a car to get back due to time of day/weather), often cheaper than owning a vehicle after considering all costs (depreciation, maintenance, insurance).

Outside a sufficiently dense city the pros might not outweigh the cons, but if walking/cycling/transit can cover most of your travel you probably don't need a car for the other half. I only have one for leaving the the city.