I'm guessing this is because plenty of speakers aren't designed to output sound outside of typical hearing range (because, I mean why bother for a TV etc)
> 3k is commonly the most sensitive frequency for human ears - we use it for testing wow and flutter on tape machines; you can hear pitch variation there easiest. 3k is a ballpark, but a good one - I think it works anywhere in that region 2-4k, but usually 3k is THE ONE.
> When I have a harsh fuzz guitar or trashy cymbal, I often try to just tuck down the 3k region, and suddenly it no longer covers everything. It still sounds thick and harsh, but nothing kills your ears.
> ...
> Cutting at 3k can help hide out of tune instruments and bad pitch on vocals as well.
