If you know your left hand chord but are playing a piano and not a keyboard, how do you get a 'basic' chord to sound anything like a piano being played? If you just hold the chord down you get the harmony, but no movement. Holding your chord down on a keyboard will give you a plethora of sound, all created from the STYLE you are using. But if you do that on a piano, the piano will sound 'dead'.
See how the demonstrator gets a harmonious 'moving' sound with chords played on a piano.
Hugh