Many people still cling to the idea that a powered subwoofer is only necessary if you are using small bookshelf speakers. While a powered subwoofer can, and usually should be used to assist in the bass reproduction of every channel, its main purpose in home theater is to reproduce the Low Frequency Effects (LFE) channel. This channel (the ".1" in a "5.1" or "7.1" recording) was introduced with Dolby Digital and DTS and offers a completely unique conduit for low frequency information and effects. If you don't have a powered subwoofer, you're completely missing out on this exciting stream of low frequency information.
The LFE channel dramatically improves the impact and emotion of scenes in all types of movies. Imagine missing out on the resonance of the approaching T-Rex in Jurasic Park, not feeling the rumble of an earthquake, or missing the low tone that penetrates your body just before the bad guy jumps out at you in a scary movie. Emotion is often conveyed through a movie's soundtrack and a powered subwoofer makes the most impactful low notes really hit home. Invest in a good powered subwoofer- you'll be glad you did.