What is the primary purpose of a dummy load?

A dummy load is essentially a substitute antenna that safely absorbs a transmitter's RF output power and converts it into heat instead of radiating it into the air as electromagnetic waves. Amateur radio operators use dummy loads when they need to test, tune, or troubleshoot a transmitter. Because the power is absorbed rather than broadcast, it allows for full-power testing without causing accidental interference to other stations on the air. The other options describe completely unrelated functions. A dummy load does not affect a receiver's signal-to-noise ratio, it cannot prevent over-modulation (which is an audio adjustment issue), and it does not improve an actual antenna's efficiency.