How can a subwoofer expand the soundstage?

I thought its because the subwoofer is omnidirectional in the lower region and thus adds to and expands the soundstage that the conventional speakers, whose sonic signature is directional, can't possibly convey. So, together, you get both directional and omni-directional cues which gives a larger soundstage beyond the mere boundary of the listening room and somewhat takes you to the recording venue instead. Unless you have already a gargantuan full ranger, most speakers can use the help of a good sub or two in this department.

Regards,

Kenobi