Why open the lid of a grand piano? To let the sound out. Unless it's a very high-grade upright piano, say a Steinway, the facings are just there to look pretty. Most uprights these days are made of woods that are usually of low/medium quality with venier glued on. It's the soundboard that makes the sound.
In the early 20th century the facings were actually carved out lace/lattice work (I think the called it grillwork) to allow the sound to escape.
http://www.moondune.com/PhotoGallery/Piano1.html
They don't make stuff like that anymore! It's just too expensive.
My father-in-law refinished pianos like this before he retired, so I had the opportunity to inspect them closely and play them. The tone of most of them was exquisite although the actions varied greatly.
On my very first piano (over 40 years ago!) the entire upper facing tilted out from the top to form the music rack and allowed the sound to escape from the sides. This was a compromise in the 20's and later to the expensive hand carving.
I don't mean to be pedantic, but spend a little time on the history of a subject. It provides valuable insights, is fun, and makes you look smart.