Subwoofer are designed to reproduce the lowest frequencies of our audible hearing range. These sound waves from the subwoofer can exceed 30 feet in length, and interact with a room's walls, ceilings, and floors. Depending on room placement, a sub creates cancellations, or reinforcements of the sub-bass. You can even prove this for yourself.
Compare the performance of your subwoofer based on room placement. Try it in the middle of the room, against a wall, and then in a corner. You should notice a marked difference in the level of bass from one location to another. Additionally, the subwoofer will sound different based on your position in the room in relation to the subwoofer.
For home theater, placement in the corner generally gives the smoothest response. For music and music videos, the subwoofer MUST be placed in between the main speakers (or very close) to avoid loosing some notes in some listening locations. Some hardcore audiophiles will use SPL meters, and advanced test equipment like RTA's to optimize subwoofer placement.