It's only really in some compact cities like SF or Manhattan with good public transport that you don't need a car. Car clubs are useful for local trips. But SV and other towns and cities you'd need one. I'd say in Manhattan having a car is a liability!

My feeling was that it's difficult to travel outside those areas?

It may, however getting a driving license is really easy compared to most countries. And rentals are not to expensive (ok these days are a bit different due to agencies selling all their cars and being stuck not able to buy new ones now).