This is called a monitor hole, and its purpose is to allow you to hear the sounds of others. Because the piano sounds through the vibration of the piano body, and the piano body is not a perfectly smooth sphere, the sound emitted will inevitably be different in all directions.
Ordinary pianos only have a sound hole on the front, which means that the front and side structures are very different, so the sounds heard on the front and side of the piano will also be very different. When you hold the piano, you hear mainly the sounds from the side, and the people sitting in front of you listening to you play the sounds from the front, so the sound you hear is different.
After opening this hole in the side panel, the front and side structures are much more like the original, so the sound you hear when you hold the piano and the sound you hear when you sit opposite are much louder than before.
Edit by Height Musical Instrument News Department