Controllable devices include Android phones, smart plugs, smart bulbs, smart switches, and more. Controllers include big-league Echo devices, like the Echo Dot, Echo Show, Echo Studio, and Echo Flex.
Why does Matter, well, matter? Once enough companies and products join the standard, you can control any Matter-compatible device from any compatible Matter app. You will never have to re-register a smart home product to get it to work with a new platform.