The other app off the top of my head is VoIP. You should be able to "dial" a number directly. Most solutions go through the company's data center first in order to pierce through NAT. Which makes it more expensive, less reliable, slower, and more susceptible to snooping.

There's a "if you build it, they will come" effect here. Once you can address hosts directly, a whole bunch of things become better, and new ideas that were infeasible are now feasible. They don't exist now because they can't.