What's the need for a coupler between PA and antenna in a transceiver design?

in a Transceiver design , in TX part there is a coupler between PA and antenna swicth...can anyone expalin why its needed ? what would happen if theres no coupler present in between

Demons would fly out of the box and inhabit the earth!

It is a measurement port.

in some transmitters u need to make a power detector , to measure the output power and make a feed back loop to linearize the PA
so the coupler , or a directional coupler used to get a sample of the transmitted power and feed it to the power detector which will convert it to DC to be measured

Khouly