The top of the ordinary world
8 kHz marks the top of the standard audiogram - the highest frequency routine hearing tests measure, because content above it contributes little to speech understanding. It is also close to the practical ceiling for aging ears: while young ears reach 17-20 kHz, hearing 8 kHz remains nearly universal, making it the "everyone passes" reference point before the age-dependent frequencies above.
In sound, 8 kHz is the heart of sibilance - the "s" and "sh" energy on vocals (de-essers target roughly 5-10 kHz), the sizzle of hi-hats, the airy edge of acoustic guitars. Boost it for sparkle, cut it to tame harsh recordings; either way the effect is instantly audible.
Testing with 8 kHz
As a tweeter check, a clean steady 8 kHz sine should sound perfectly smooth - any crackle or intermittence indicates a failing driver or a dirty connection, and it is high enough that room acoustics barely interfere, so what you hear is genuinely the equipment.
Because directivity narrows with frequency, 8 kHz is also a fine test of speaker aim: move off-axis while it plays and hear the level fall - a practical demonstration of why tweeters should point at your ears, covered in the studio monitor setup guide.