The bookshelf or small tower speakers in this country are commonly only going down to between 40 and 60 cycles. That leaves at minimum an entire octave of music missing from your listening experience. To fill in the octave of missing information, a subwoofer is used. Well, it is an issue of localization. The pure objective of a subwoofer is to make your present speakers sound like they have bass, NOT to sit over in the corner and draw attention to itself as it booms away.
Different frequencies have different distances between wave fronts.
For example, a 1000 Hz tone generates sequential waves spaced around 11 inches apart. The higher the frequency, the shorter the distance gets. The human ear was designed and shaped the way it is, to triangulate these frequencies and tell the brain where the source is located. That way you know which direction to run when you hear a noise in the middle of the night. If the noise you heard were lower than 100 Hz, you would really panic, because not only would you feel the noise, but it would come from all around you.