Users can enter the destination before being picked up. I've had drivers cancel before arriving, so I usually wait to enter the destination until my seat belt is buckled.
While this is true, it's pretty easy to infer from gps. If there's a 10 minute ETA and you just went over the proverbial train tracks into the bad side of town, or you're familiar with the surrounding towns, it's not difficult.
It lets you get a fare estimate and it lets you avoid having to spell out the destination address to the driver, so it's easier on both of you.
This is especially true when the address contains difficult or uncommon words like Sycamore or Cheswycke, which I'm pretty sure I'd screw up writing, and I'd guess that others would as well.