This especially makes sense for people who have a regular commute to the airport etc. I've experienced the same thing with Airbnb: first stay via the platform, and further stays booked directly. With Airbnb you miss out on the platform's protections. But with Uber, as the drivers are getting paid less and less, it seems easier to slide into direct contact.
It can be useful to have a private car driver you know and trust, without the anxiety of the wait, or of drivers calling you to ask where you're going, only to cancel the ride. (This happens to me regularly at the airport).