There are 3 reasons why they may want to buy a smaller subwoofer:
1) Can't afford an 18
2) Don't have the space for an 18
3) Don't want an 18 for aesthetic reasons. (The silliest reason of all, but ok...)
Let's talk about each one:
1) If you are down to your last package of ramen, then you probably shouldn't be buying AV gear at all (or cigarettes or alcohol or lotto tickets etc).
2) Is what it is.
Most 8's can fit in 8x8x8, and many 18's can fit in 18x18x12 (sealed min-box).
If you use low-profile drivers, even smaller.
Will it be as good as a 30cuft horn? Well no...
3) Do whatever you want dude. But evidence has shown that bigger drivers perform better at bass, with few exceptions; and 18's are in the Goldilocks-zone. (Maximum Boom/SQ, per dollar.)
Most of the richest people I know, are also the stingiest/nastiest people I know.
Like all the people on my street, pretty much. They are mostly all, millionaires, or well-off at least...
(Unfortunately they didn't get rich by being Mr.NiceGuy or giving their money to the disenchanted.)
Do 8inch subwoofers work? Yes
Is it a good idea? Depends...
Is it the best option/deal/boom-per-dollar? Probably Not!
Is it the best driver that exists? Hell No!
Is it at-least an acceptable driver? If you say so...
1899's used as Atmos Heights are highly-impractical in every regard, although it is close to "Best-possible" as far as heights go.
The-Best is rarely the Most-Practical.
They have been lying to you, we have been able to get a human to Mars within 7 days since like the 60's now, but the rocket acceleration will kill you within 30 seconds. The thrust alone will turn you into a pancake before you even leave Earth-orbit.
But without all that air "slowing" it down. hehe!
Bro: We got "all the speed you can withstand", and then some...
You think your commute to work is bad now... just wait until 2030 when all the mouthbreathers are all driving Tesla's, everyone being capable of 0-to-60 in 1 second with a top speed of 400mph. (This will not end well!
Not always practical, but all the chocolately/warm/detailed/tight/loud/fast that you can withstand.