Technically, you can’t really use a MIDI keyboard controller as a digital piano since they don’t play any sounds themselves. Instead, they send signals to a computer or other device. Those signals tell the computer which notes to play, and the notes produced are then sent through the inbuilt speakers on the computer or keyboard, or a set of external speakers. So, though the MIDI keyboard controller is triggering digital piano sounds, it isn’t the same as using a digital piano.