It seems to me that the usefulness of PTT is that it replaces walkie talkies and dispatcher/fleet/serviceteam radio networks. A new or growing company that needs to do that kind of thing no longer has to invest in all the radio equipment, it can just buy the right brand of cell phone for its employees.

Of course, this gives the cell phone companies a chunk of a market that they didn't have before... that's the *true* usefulness of it.