But I have a HomePod mini that I use for all of my Matter devices, why would I need a massive device with a screen just for that?

The only difference I can see is multiuser support of which Siri works just fine for me controlling the matter devices in the house that are on my partners Apple account. I can't justify buying a product just for multi user support and have another bulky device sitting in the house.

Apple would've been better off putting a FaceID camera in a Homepod mini or Homepod to get the same functionality.