I can answer that.
Subs add both low frequency reproduction as well as additional power in that range which most full range speakers either struggle to provide, or struggle to provide without eating amplifier power, as mid range speaker cones are designed for mid range reproduction.
An active (separately amplified) sub adds a number things - more power at low frequencies to match what is usually available in the midrange, a way to relieve midrange speakers of the burden of reproducing frequencies at the limit of their range, a way to use a main amplifier to drive the mid and upper frequencies only, making the whole system effectively more powerful if you care about that. And if you plug your music system into your telly you have an instant home theatre system.
Dance music benefits since is designed to have low frequency components below 45Hz, but most other forms, particularly those containing percussion like kick drums, gain a definite feeling of presence with a sub, it seems to fill the room more convincingly.
If my sub broke tomorrow, I would go straight out and buy a new one. When someone plays me their fantastic newfangled music system, my most regular comment is ‘When are you getting a sub for it?’
The only exception would be vinyl based nostalgia music systems, since there is less need for low frequency sound that vinyl doesn’t reproduce well anyway.
Your scepticism is understandable. If you’ve never had a sub maybe you don’t notice the difference, but if you’ve ever had one you won’t want to be without one in my view.