Earlier quoted context omitted.
To get from point A to point B without walking half a mile and waiting 10 minutes for a bus to go a quarter the speed to a stop half a mile from your destination. I'm sure a lot of people want a car so they don't have to expose their groceries or small children to the disgusting mess that is public transportation in the vast majority of (American) cities.
The comment I was responding to was saying everyone wants to own their method of transportation. I'm not asking why you would want to drive - I'm asking why you would choose to own a car if there was an alternate world where all the other cars in the city were available to use on demand.
Basically, a large operation could have economies of scale (in house mechanics, only two models of car to choose from, own the fuel stations...) but the utilization of the vehicles probably would not change dramatically from the utilization of the private "fleet" we have today.