What is primary innovation that a piano delivered over a harpsichord?

Since a harpsichord plucks the strings the same way each time in response to a keystroke, it delivers the same loudness for each note. A piano on the other hand uses padded hammers to strike the strings, and allows the musician to control each strike in such a way that they have control over the dynamics of each note. This control allows pianists to express a broad range of emotions by adjusting the loudness and softness according to the feelings they want to convey to the listener.