Why 1 kHz is the reference frequency
1000 Hz sits near the center of human hearing sensitivity and comfortably inside the flat region of virtually every microphone, amplifier, and speaker. That makes it the natural neutral ground for measurement: sensitivity specs (dB SPL at 1 W/1 m), THD figures, and dBu/dBFS level alignments are all conventionally quoted at 1 kHz unless stated otherwise.
It is also the anchor of the loudness weighting system: the phon scale defines loudness levels by comparison to a 1 kHz tone, and the A-weighting curve used in sound level meters passes through 0 dB at exactly 1 kHz. In broadcast, a continuous 1 kHz sine is the standard lineup tone sent ahead of program material so every stage of the chain can be set to the same reference level.
Practical uses for this tone
Level matching: play the tone through your chain and set each device (interface output, mixer channel, amplifier) so meters read the same reference - the classic use of a lineup tone. Gain staging a recording chain with a known 1 kHz signal makes clipping points and headroom explicit.
Quick quality checks: a clean 1 kHz sine reveals rattles and buzz in speakers at moderate volume, audible distortion in failing amplifiers, and channel imbalance (combine with our speaker balance test). For distortion measurement proper, feed the downloadable WAV through equipment into an analyzer or DAW spectrum view - harmonics at 2 kHz, 3 kHz and above are the distortion products.
The tone above is synthesized at your sound card's clock accuracy - typically within a few parts per million of exactly 1000 Hz.