Speakers interact with the room they sit in — corners and walls reinforce certain frequencies, especially bass — and a quad equalizer offers a simple way to compensate.
If bass sounds boomy or one-note, a cut around 60Hz often helps more than it seems like it should. If the sound feels boxy or congested, a small cut around 500Hz can open things up without losing warmth.
EQ can compensate for a room's acoustics but can't fully replace good placement — speakers pushed tight into a corner will usually need a stronger bass cut than the same speakers pulled a foot away from the wall.