I thought I explained it, but perhaps not clearly enough so let me try again.
First of all, when you have floor standing speakers they usually don't occupy any more floor space than bookshelf speakers on stands. In the case of mine, they actually use less space than the stand mount speakers they replaced.
Floor standing speakers have a larger cabinet that becomes part of how they perform. With a larger cabinet the builder can effectively create separate spaces within that have the desired volume and porting matched to each individual driver. Or they can make one big resonant box, or something in between. In general terms, there is no replacement for displacement in automobile engines nor loudspeakers. A typical floor standing speaker can generally achieve lower frequencies than it's bookshelf counterpart.
Now we get to why get a floor standing speaker if you have a subwoofer. I'm a guy who uses the system mostly for music. Much of the music I listen to doesn't have anything below 50 or 60hz and it's really nice to switch the receiver into direct mode, sending the full signal to my speakers and nothing to the sub. The music sounds cleaner, smoother and presents better stereo imaging. I appreciate that sound so much that I will someday get better tower speakers that have much larger drivers and can go even lower.
Not a lot of 2 channel integrated amps or receivers have true bass management to control a subwoofer, even the high end ones. The more I can depend upon a single pair of speakers to present the music, the better it sounds. Until then because I do also listen to some modern music that will shake the walls and watch some movies that so the same, I need a subwoofer and use it.
Floor space saving is the only legitimate argument I see here for using towers over bookshelf speakers. If your space is really tight, yes, tower speakers have less of a footprint.
However, regarding separate spaces within the cabinet, that is not something that isn't being done with bookshelf speakers. Lots of higher end bookshelf speakers have separate compartments for their drivers. Plus, panel resonance is much easier to control in bookshelf speakers since the cabinet panels are much smaller. Furthermore, regarding 'no replacement for displacement', a bookshelf speaker and a decent sub will have much more displacement than the tower equivalent of the bookshelf speaker. A tower speaker might have more headroom in the mids, if it is a three-way speaker, but the limits of such a speaker is more likely going to be in the lows, and a reasonably good sub will almost always provide superior bass performance over a tower speaker, unless that tower speaker is very large and expensive. If you are listening so loud that dynamic range in the mids become a problem for your bookshelf speaker, chances are your tower speaker equivalent will not be up to the task either.
Also, the best placement for a speaker's bass response in-room isn't always going to be the same place as the upper frequency bands, in fact, it rarely is. For the best performance, a modular setup is needed so that the subs can get a flat response at the listening position and the upper bands go with the loudspeaker's recommended placement. You can not just plop the bass speakers arbitrarily in room and expect a good response, as is the case with tower speakers. Multiple subs using careful placement will always provide a substantially better sound, doesn't matter if you are listening to two-channel, or surround sound, or mono-aural radio dramas from the 1940s.
As for not a lot of two-channel integrateds or receivers not having bass management, that is a argument against the electronics not having bass management, not an argument for tower speakers. Such designs decisions are antiquated notions of stubborn old audiophiles. I am arguing for a better sound, not catering or compromising for a lesser bass sound. And this even goes for music tastes which do not have content below 60 Hz, because the Schroeder frequency is very likely to be well above that point.
