Why Does a Grand Piano Action Need Escapement?

When a pianist depresses a key, the finger does not push the hammer all the way onto the string. The key drives a system of levers that accelerates the hammer; just before impact, the jack must slip out from beneath the hammer assembly. This is escapement. If the key continued to hold the hammer against the string, the string could not vibrate freely and the contact could become dull or unstable. Escapement lets the hammer fly through the final distance under its own momentum, strike, and rebound at once.

A modern grand action also uses a repetition lever to catch the hammer before it falls to its lowest resting position. The key therefore needs to rise only part of the way before the action can prepare another stroke. Rapid repeated notes and tremolos do not require the finger to release the key completely. The double-escapement action developed by Sébastien Érard in the early nineteenth century became one of the foundations of the modern grand action.

What the player hears is not a mechanism but a sequence of distinct beginnings. In rapid repetition, lifting the finger too high wastes motion, while continuing to press at the bottom of the key does not prepare the next note. The useful sensation is the key rising to the point where the action re-engages, followed by a fresh impulse. Escapement does not make the music for us; it builds release into touch, so that every new sound depends on allowing the previous stroke to finish.

https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/mechanism003.html
https://www.metmuseum.org/art/collection/search/503046