Why the fascination with subwoofers?

Subs are simply just another very flexible and cost effective way to get extended bass when needed or desired , often in combo with smaller speakers with less bass extension, of which there are many very good quality affordable ones otherwise.

I use a fairly compact but extended 8" sub in one room which has tight quarters and with very high quality smaller speakers with limited bass extension that benefit.

I don’t use one, though I could, in others where the larger main speakers used do extended bass quite well alone.

I also don’t use one out on the deck where I run another pair of good quality smaller outdoor speakers with limited bass extension, mainly because I do not care about bass extension out there.

So like most things it all depends. Many ways to skin the cat, if needed.

@akg_ca 

The one big problem with all of this is that you need a good crossover to roll off the deep bass in your system and achieve all of these benefits...”

So, I gotta ask, where do I get that crossover?
Vandersteen makes their M7-HP, which looks like a high quality unit (pricy) and I would be interested but unfortunately the 100 Hz high pass is higher than the (40-50 Hz range) frequency that I would want.  Also, based on the images of their unit, this seems to be a bit more complicated than an in-line capacitor so maybe not achievable as a DIY project.

Back to the OPs question, I would look at one sub for an HT set-up and two or more for music.  By using mains with excellent sounding bass that is limited to a cut-off at about 40Hz, and then rolling in subs for their greater control at the lower frequencies below 40 Hz, I have eliminated low frequency room interactions and greatly increased bass drive. 

Here is one very affordable option that I would consider if needed:

https://www.amazon.com/gp/product/B00102VWJK/ref=ox_sc_saved_title_1?smid=ATVPDKIKX0DER&psc=1

Generally people would be probably be better off using vibration isolation for the front end, in terms of getting more detail and slam in the bass as well as avoiding interference in the midrange. There are many ways to skin a cat.

1) Most speakers get to maybe 40Hz at reference levels. Even if they state say 30Hz +/-3dB, that’s at only 1 watt, many will dynamically compress.

2) Many songs go down to 30Hz or 35Hz. Some music with pipe organs go to around 16Hz. Movies have gone as deep as 10Hz.

3) Alleviating the bass from your speakers and amplifier(s) will ease their burden, thus usually allowing them to get louder, and thus be more dynamic.

4) Subwoofers have far more headroom, thus you can do a lot more DSP on a subwoofer at 60Hz than you can for a speaker at 60Hz.

5) The best place for baseball in your room is most likely not where the speakers are, having subwoofers allow you to find their best place in the room for where you sit. Of course, if localizability is an issue, keep them near the speakers.

6) People over-estimate how much bass they currently get. I was content with the bass from my cheap 8” sub, I thought it went decently deep and went decently loud, but I was so wrong, once I got a decent 12”, it was night and day, more of a difference than upgrading from $400 speakers to $40,000 speakers.