Earlier quoted context omitted.
It's obsolete and only needed because their software is broken. Skype, Google Hangouts, teamspeak, ventrillo, mumble, and tox all support my microphone and voice activation correctly.
PTT is useful when you don’t want everyone else on your channel to hear your breathing, coughing and keyboard-clacking (can you tell I’ve played Overwatch?)
The rise of mechanical keyboards has made PTT practically the only option. Otherwise you just hear little clicking all the time.