Why Does a Grand Piano Lid Open at Different Angles?

A grand piano's soundboard radiates the strings' vibrations into the air. The lid does not create the sound, but it changes how that sound leaves the instrument. When raised, it acts as an inclined reflecting surface, directing some upward-radiated energy towards the open side of the piano. With the lid closed, more high-frequency sound is obstructed, and the result is usually less direct and less brilliant.

The full-length prop raises the lid higher and is often used for solo playing or when the piano needs to project clearly into a hall. The short prop reduces the opening and may suit chamber music, accompaniment or a smaller room, where the piano could otherwise overwhelm strings or voices. Closing the lid is not equivalent to turning down a volume control, because timbre, direction and the balance heard by performer and audience all change together. Microphone placement can magnify these differences in a recording.

Listen for whether the instrument remains bright nearby yet blends at a distance. The direction of the lid, the position of the ensemble and reflections from the stage all affect the result. A pianist cannot judge the whole hall only from the bench. Lid height is part of the acoustic arrangement: it is one way the piano negotiates its place with the room and with other instruments.

https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/
https://www.britannica.com/art/piano