Subwoofers are designed to add the low bass frequencies that small desktop speakers, in-ceiling and in-wall speakers can not produce.
Most subwoofers and active which means they have their own built in amplifier and take no power from the amplifier that drives the smaller speakers.
Many models can plug into the Hi-Level speaker terminals of the amplifier or low level audio signal line out of an AV receiver or music device.
Because frequencies below 80Hz are non-directional it is very hard to tell where the bass is coming from which makes subwoofer placement very flexible.
Level controls and crossover point controls mean the subwoofer can be integrated with the audio of the smaller speakers to produce a full range rich sound.