Bass is where the foundamental frequency of human voice and of many musical instruments lies. The mid and high frequencies give you the overtones and the directional cues. You must have both in perfect harmony to attempt to fool you brain into thinking you are actually listening to a live performance.  A bass driver is large and has a large cone excursion, therefore  it requires more power and more control than the other smaller drivers. A powered subwoofer is directly coupled to the bass driver as the crossover is positioned before amplification, unlike a passive speaker that has the crossover between the amplifier and the bass driver. Additionally, all the power of your amplifier or receiver is now used to drive the mid and high frequencies. The sub has its own amplifier which greatly reduces the strain on your receiver.
You do not need to understand it, but your ears will.
We currently have 13 different subs on display, ranging from $300 to $8,500, one of them matches yous speakers perfectly!