I've never owned a subwoofer. I primarily listen to acoustic music such as classical, jazz, and film scores. I realize that a subwoofer is probably important for genres that feature heavy electric bass and kick drum. But how useful is a subwoofer with natural room recordings of acoustic instruments?
I'm an absolute minimalist so I hate subwoofers with a passion. I hate the boxes, the extra cabling, extra dedicated outlets, the more cluttered look, all of it. If you have true full-range speakers say down to 23Hz or so, then your chances of dialing-in your speakers or acoustically coupling them to the associated room to achieve a musical bass can be far easier than having to introduce 1 or more subs. But of course this requires much work.
If you lack full-range speakers or unable to fine-tune your full-range speakers, then that's where a subwoofer comes in. But integrating and fine-tuning the subwoofer to the main speakers can require even more work to achieve a musical bass.
How useful is a subwoofer? Well, I think a better question is, how useful is a musical bass in a given playback presentation whether it's from pair of well-placed full-range speakers or well-placed full-range speakers plus a well-tuned subwoofer?
To that, my response is, it's invaluable and regardless of genre. It's not just a more complete, tighter, deeper, more well-defined, and more musical bottom end, but rather if done right, the entire playback presentation benefits immensely. Without such bass the playback presentation will sound lean, overly bright, unbalanced, etc but with a subwoofer done right, the overall playback presentation across the entire frequency spectrum is that much more musical, richer, fuller, pleasant, natural, and just plain more enjoyable. But much patience is required. Many will invest an hour or 2 in fine-tuning their speakers and /or subwoofer and call it good when it ain't. Now I don't do this full-time but I find acheiving this level of musicality requires, weeks, months, and sometimes even years to really get things dialed in.
But if you can't get this with level of bass with just your speakers, then a subwoofer is an absolute requirement and your ears will thank you if you invest the time to do it right.