Stuart and Sons have invented a huge 102-key grand piano. What is the reason behind this?
The maker has said in an interview (link from replete’s comment):
I'd hate to go back to the 88-key piano... I couldn't stand it. It's too limited.
Which suggests he values having the extra notes.
He also said:
I'm all for innovation in the modern piano. To me, the piano is a pinnacle of human achievement. So it seems right and proper that it should continue to develop.
Which clarifies he is in favor of finding ways to expand the capabilities of the piano in general.
Note that while organs and synthesizers don’t always have keyboards with as many keys, they generally can both be played higher and lower than typical pianos, so it’s not like an 88 key piano has all the notes we would want to play or hear.