The long continuous hinge on the lid of a piano is so characteristic that it's even named a "piano hinge" after this use. But why do pianos have such hinges, when a set of separate hinges, such as you'd find on a wooden cabinet or jewellery box, would be cheaper to make and easier to fit?
Is it just to make it look nice as a premium piece of woodwork? Is it to increase the rigidity of the structure or prevent unwanted vibrations in the joint? Or is it for some other reason to do with the sound or the construction of the instrument?