I use my bicycle every day, but some people don't have that option. Also I normally use Uber more than once a month. The thing is, a car is way too much of hassle to use most of the time in the city. You need to park it, there's traffic, and then you can't drive drunk or so I've been told.
A taxi service is an essential complement to mass transit, and Uber provides that service in areas with poor taxi service (essentially everywhere).