There are all kinds of technical reasons, but the ultimate answer is, What do you hear, or miss?
Go to a live performance of chamber music and listen to an acoustic bass (instrument). Chamber, because there is more chance to hear the bass by itself. Acoustic, to get a direct feel for how an unadulterated bass instrument sounds and feels.
Then listen to similar music on your system. You will hear the bass differently. If the bass lacks depth, or floats strangely around the room, *and if you want to try to improve those shortcomings*, then you might try subs.
Cheers.