The piano action's overall goal is to perfectly translate every aspect of the player's finger and hand movements...
A MIDI piano seems to be typically an electronic piano keyboard, sometimes with other wheels, buttons, as well as sliders, used only for sending signals of MIDI as well as commands to many different music devices or computers via a MIDI cable of 5-pin or USB.
Chords describe the song harmony, and much more than a single note is played simultaneously. They could be straightforward or complicated, and that even beginners can build exciting and fun music. Indeed, very rare songs, in particular in rock songs, don't even use chords in any way.