Many SES Units have a few ear-piece's and some have remote mikes. In a noisy environment (V8 race concert, chainsaws, flood pumps) they are the only way you can hear the radio.
Because of the cost (Unit charged) of each ear-piece, they are normally issued to Team Leader & Incident Controllers.
They have also been used by crew that are monitoring relatives at a life threat situation (eg land search). Then the relatives cannot hear the radio conversations.