Why A dummy load is a non-inductive resistor, usually 50 ohms, that presents a matched load to the transmitter and turns the RF output into heat instead of radiation. That lets you tune, test, or measure power output without putting a signal on the air where it could cause interference. It also protects the transmitter by giving it a proper 1:1 SWR load during testing.
Watch out The choice about antenna efficiency is backwards: a dummy load is deliberately designed not to radiate, so its radiating efficiency is essentially zero. Modulation level and receiver signal-to-noise ratio have nothing to do with a resistive load.
Dummy load = 50 ohm resistor that turns RF into heat, not radio waves. Test without transmitting.