The turning into a pile of rubble was gradual, there was something worth defending not that long ago. But mostly I suspect it is because they have to kill the R

The turning into a pile of rubble was gradual, there was something worth defending not that long ago. But mostly I suspect it is because they have to kill the Russians somewhere. If they have a strategic advantage defending the pile of rubble they will cause more damage to the invaders than they will take themselves. Who ultimately holds the rubble is less important than what the battle cost each side.