Floating floors reduce mechanical transmission of vibrations (i.e. sound) from the outside world- particularly low frequencies. One studio that I work at is right next to a road. In the office outside the studio you and *feel* a truck rumble by. In the studio with floating floors you can't- and it doesn't make it into the mic. Without the floating floor, things like doors slamming pretty much anywhere in the building can be mechanically transfered through the floor, up the stand to the mic.
It also decouples the floor from the walls, since you can't both seal and decouple the walls from the actual floor. That way your bass amp doesn't rattle the building and make it into the vocal mic in the iso booth. Or your neighbors house, for that matter.
They aren't NEEDED, but they are part of standard studio solutions to sound treatment and sound proofing.
Take care,
Chris