Imagine having a recording studio where the floor space is a premium... and we want to "stack" these into the corners. The room is already cluttered but we're fixing that... so having the subs under the mains in the corners would be out of the way enough to justify having them.
We need the lower octave because we run a few vintage analog synthesizers that need to "push" in the mix.
it's possible the subs may have a spot along the same back wall not directly under the mains... but the mains are going up in the corners against the sides and back wall... at head level... so the subs can't go back any further.