Higher frequencies you may think are unaffected by a subwoofer, because they have no bass. Actually every note comprises a fundamental frequency and harmonics which comprises a much wider frequency spectrum than the fundamental note being played. This is why a high note played on a piano sounds different from the exact same high note played on a violin, which sound different from the same note on a flute, clarinet, oboe, and so on. The hardest thing for a full range speaker to reproduce accurately is the lower frequency bandwith.
Ever notice how a large open building like a cathedral or concert hall has a sound? You can sense you are in a large building. They have a certain ambience. A good musical sub will fill in the lower frequency spectrum of the instruments playing, as well as the room ambience you can feel, as well as hear. High notes become clearer when the whole sound is reproduced to the fullest.