The subwoofer speakers on your sound system are designed to produce low pitched sounds.
With a subwoofer, you can be sure that you'll experience all the low pitched sounds in a game, song or movie.
If you have a system with a subwoofer as part of a surround sound setup, then you should expect a full theatre experience when you watch movies on it.
If, you're favourite genre of movie is horror films for example and you purchase and/or download a horror movie to your computer.
If you've purchased a highly quality sound setup with a subwoofer, then you should expect to hear the small creaks that are made while a character walks down the stairs, which would naturally add to dynamism of the scene.
Obviously, no one is silly enough to out and purchase a subwoofer by itself.
It's important that anyone interested in purchasing a subwoofer, buys it as a part of a surround sound bundle.
It's also important that before you go out and purchase your surround sound speaker setup, you evaluate the size of the room that you intend to have your system in.
If your living room or bedroom is fairly small, then an Altec Lansing BXR1221 system should suit you nicely.
However, if the size of the room that you intend to watch and play music in is fairly large, then the Logitech Z2300 will best suit you.
You should also consider amplifiers when you purchase your subwoofer system.
Active units are self powered, this means that they come with their own amplifiers; which is something that you should definitely consider before buying.