Uber is an "ON-DEMAND" service, which is the real beauty of it. You walk out of a building and it's there... When I contact any of my usual hire car drivers, they are rarely "around the corner". They are often on other jobs, or across town, in which case my fare is less attractive and I have to wait for them to drive all the way across town before I start my journey.
If you are planning a specific series of events in advance that have a firm time (e.g. meeting finishes at X, or an airport run), it is usually best to book a hire car directly in advance.
But Uber is great when you don't have firm requirements in advance (e.g. going home, or social events, or anything when you don't know exactly when you want to be picked up).
I think there are also a number of people who have used Uber as their first hire car / VHA experience, and have realised that limos are better than taxis and not substantially more expensive.