Every headphone model has its own built-in tonal bias — some are bass-heavy, some are bright — and a quad equalizer lets you correct for that directly.
Consumer headphones are frequently tuned with extra bass and treble and a slightly recessed midrange (a "smiley" curve) to sound exciting in short demos. If vocals sound distant on your pair, try a small boost around 500Hz–2.5kHz to bring them forward.
In-ear headphones tend to run brighter due to their close seal, so a small cut at 8kHz is often welcome. Over-ear headphones vary more by design; open-back models usually need less bass correction than closed-back ones.