I think the idea is that it stops the cracks propagating. If the tiles can move individually then it means that one crack doesn't spread
I suspect the insulation value of the new floor is less than the old one given the large amounts of wood with insulation between. The old floor insulated everywhere given how the polystyrene was laid.It would have been much easier to just lay laminate on top of the old floating floor, and probably warmer too. Why the insistence on a tiled floor?
Err, wife wants tiles...
Wait until someone drops something and cracks a tile. Then see if she still wants tiles.