Answering the first part of the question, "Why subs?" - because music can include frequencies in the human audible range, 20Hz to 20,000Hz. Most speakers (towers and otherwise) can't play as low as 20Hz; therefore, as a listener, your potentially missing part of the music. We can't hear what the speakers can't play. Would you fully enjoy a painting if it was missing some colors? No, it would be incomplete and likely unbalanced. Avoid the necessity of subs by choosing "full-range" speakers. Why companies make floor-standing speakers that can't play down to 20Hz is a mystery to me - who could possibly want this (unless they intend to add subs in the original plan). To me, these companies are like artists intentionally painting incomplete pictures... a 'novelty item' - no thanks.
The second major issue is how low frequencies (long wave lengths) are affected by physical room dimensions. Very few of us are listening outside in the middle of clear open space. Instead, we are typically listening from inside a box, and those walls, ceiling and floor create peaks and troughs of interesecting air pressure from reflections. These bass nodes change the sound (distortion). How to solve it? Room treatments and multiple sources (multiple subs spread throughout the room) - the more subs the better. Not for more bass volume... for more linearity throughout the room. Additionally, higher quantity of sub drivers (more subs) allows each individual driver cone to move less while summed together achieving the same volume. Less excursion means more accurate cone movement ("fast bass"). Again, more subs is better, but beware - like anything else, there is a curve of diminishing returns regarding value.
