When is the addition of a subwoofer useful?

An addition of a subwoofer can be useful if:

– your setup only have space for smaller monitors and you want to extend the frequency response to full range.

– you want to have higher SPL/dynamics of your monitoring system.

– you need more lowend energy for modern/electronic music genres.

– you want to have more flexibility to move the subwoofer within the room to counter room modes.