Subwoofers bring sound into the tactile realm, making the sound something you FEEL. They are commonly used in home theatre applications (especially with satellite speakers) to greatly improve bass performance.
Sound produced by a subwoofer is less directional than that produced by full-range speakers. However, proper placement is still necessary to get the best result and avoid standing waves ('boomy' sound). Advanced subs will feature a comprehensive set of adjustable parameters for calibration to suit the acoustics of the room.