The Subwoofer is an essential component of any Home Theatre system. It is often associated with the deep rumble of explosions and earthquakes, and it gives a greater sense of realism in reproducing such effects.
Even large floorstanding speakers are generally unable to produce the very low sub-bass frequencies that a subwoofer will produce. This is because forcing a loudspeaker to produce these frequencies causes significant vibration and will often compromise the speaker's performance in the midrange and treble. Therefore separating the sub-bass frequencies to only be produced by a separate driver makes perfect sense.
A good subwoofer, if set up properly should be unnoticeable until it is switched off.