There are at least two mediums by which sound moves around a room. The first and most obvious is in the Air. However, most people don't realize that sound actually travels better through solid objects than it does through air.
So, if your Sub is in tight contact with the floor, especially wooden floors, then sound is going to be driven down into the solid floor and into adjoining spaces.
Relative to sound in general, of course, Bass it what most easily penetrated between spaces.
If you Sub is in contact with a wooden floor, then the wooden floor, as is found in most houses, will vibrate or resonate, and within a certain context amplify sound in adjoining spaces. Lifting the Sub up off the floor can stop at least this one path of sound transfer.
When an object like a Sub is in tight contact with the floor, that is called Coupling. When the object is separated from the floor, that is called Isolation.
Concrete floors don't have much give to the, so in this case Coupling is probably the better choice. In an older building with wooden floors and shared space, Isolation is probably the better choice. But keep in mind, Isolation only reduces one path of sound transfer.
Any air leaks between spaces, such as pipes or electrical, can allow a path for sound to transfer. If you are trying to create a sound proof space, as an illustration, you have to find a compromise between creating a completely sealed room and having sufficient air exchange so you don't suffocate. Even the transfer of Cables between a recording spaces and the studio control room takes very considerable effort to make sure that doesn't create an air leak, and thereby a sound leak.
So, the purpose of isolating a Sub from the Floor is to prevent sound from being driven down into the floor where it will more easily transfer into adjoining spaces.
Just a thought.
Steve/bluewizard