Take an educated guess then Nick.
It’s so they can sell floating vented rotors for £40100 each in pretty colours, rather than plain steel for £20. Same reason they sell braided steel hoses.
Hope Vented rotors have been around years, they came out when everyone wanted the biggest most pointlessly strong brakes around for no reason. Very shortly after everyone went to small calipers and tiny rotors on DH bikes to save weight and the world moved on. They still exist because they can still sell them to people who think they need them, or more often, like the look of them. I wouldnt be amazed if 50% of them ended up on over-blinged fat bikes used to ride around central Tokyo.