pseudonym58 But perhaps there are a very few pieces that include those notes.
I think all of these are parts of the answer ShiroKuro is looking for.
Yes, when first created, the extra notes on the Imperial were added due to a request from Ferruccio Busoni, for the purpose of transcribing some Bach organ works that needed the additional range (think of all the extra notes on the multiple manuals and foot pedals of a pipe organ!). So there are indeed a few works that use the notes specifically.
But how many? There simply aren't a lot of Imperials out there (or any other >88 key pianos at that). So you really aren't going to find much in the standard repertoire that calls for them. Or for the extra high notes in a Stuart 108, etc.
And if the notes are so rarely used, you have to balance the fact that many pianists who are used to 88 keys can be visually distracted by sitting at an extended keyboard. Nobody thinks about it, but they actually orient their hands based on the size and position of the full keyboard, and when exposed to more keys, some pianists find that they subconsciously assume that C0 is a normal A0, and overstretch. Hence the cover or the ebonized keys - it makes it easier to ignore them unless you are specifically playing a work that needs them.
A lot of people say notes that low aren't particularly musical, so to @pseudonym58's point, just having them there for additional soundboard area and damper-up resonance can add a lot to the body/character of the sound. But there's also the case of improvisational playing - if you want, you can take an arpeggio that ends with C1 and just extend it down to the new lower range, and that can be a lot of fun, and a natural way to use the extra keys for normal pieces!
