All the transmitter RF feeding out to the antenna passes through those ground screws at the corners and rear edge of the circuit board, carrying it to the aluminum frame, and then to the body of the antenna socket.
The factory routinely ships these radios with the antenna socket loose, the ground screws loose and the side-mounted mike sockets loose.
A new radio brought in to be "turned up" would get all those fasteners tightened before we give the radio back to the customer.
If someone has done this to the radio already, that's good.
If not, this prevents the ground side of your RF power from reaching the antenna socket.
73