A subwoofer is a speaker designed to reproduce very low bass frequencies where sounds are often felt as much they are heard. The subwoofer is used to provide the powerful audio rumble and punch commonly found in movies and music videos, in particular for dramatic effects associated with crashes and explosions.
Even though your primary (left/right) speakers may have an impressive bass response, they won't be able to reproduce the very low frequencies delivered by a good subwoofer, most importantly not at an adequate power level. Home Theater is about audio immersion and getting involved in the atmosphere created by the surround sound system, a subwoofer is an essential component of the total package to achieve total immersion.
