What does a subwoofer solve?

When I listened to my system without a subwoofer I was pleased and could clearly hear stereo separation. But I had heard other systems whereby I heard much deeper frequencies in the music. In those systems I could not always place the lower notes/sounds/instruments but I was surprised I heard that musical information I was missing on my own stereo system. Once I bought one, then two subwoofers I had that same effect. But I do agree, at some point, usually when one spends over $5 to $8k you can get true full range sound from stereo speakers. I do agree with many people commenting here that proper integration with sub and speakers that play well together is key.

If you mean,’" What potential problems can a sub-woofer solve with regard to sound?" I would say many. And I would say in my own system, when properly integrated they supplement the low frequencies and allow me to hear lower frequency sounds. They add and enhance those lower frequencies because the sub-woofers I had chosen are flexible and designed to do just that. They can add lower level detail and allow my speakers to simply produce the frequency range they are designed to reproduce. I also think the allow my amplifier to work easier. in the end it’s a matter of your flavor of equipment and sound. Sub-woofers are just another way to get quality sound. But not always the best way. BTW even full range speaker systems have been known to benefit from quality sub-woofers. IMHO