Customers use Uber because there are enough drivers that service tends to be reliable; Drivers use Uber because there are enough customers to sustain a living.

It's the usual marketplace model.

A concurrent could book a Uber ride for their customer using the API when and where they don't have any driver.

Well, I checked the terms of use of Uber API and competing with Uber is forbidden...

People telling each other about Uber. Mass media writing about Uber. People working for Uber.