It does help bandspace availability. If you were on AM, it would be using both the upper and the lower side bands, so you'd only be able to do one thing at a time where, by using both upper and lower for separate things, two people could be on the same frequency and be using it for two different purposes without interfering with each other.

Mind you, the receiver has to be very accurate about being able to measure the side bands, otherwise you could have bleed over. I'm assuming the old tube radios would have problems with it versus today's crystal oscillators and such.