Some manufactures of exotically large speaker systems and even some of those who include self powered subwoofers within their speakers often display them with outboard subwoofers. The location of the speakers optimized for the listening position is quite often in a rooms null which can choke the low frequency response. Adding outboard subwoofers in the rooms modes is the solution.
A host of individually unique room issues can easily be solved depending on the flexibility and level of control of low frequency processing which is not generally focused on by this community.