When earbuds are tuned perfectly to your hearing, you’re able to hear a large amount of detail in the music that you’d otherwise miss out on.
Every instrument is more distinct and present, and you feel much more like you’re actually there, listening to a live performance.
Most earphones are tuned to the hearing of an average listener, but not to the subtleties of each individual’s hearing.
These subtleties make a big difference. Normal hearing varies significantly from person to person, and even your left ear has different sensitivities to your right.