Whites vs Blacks

Please help me to understand the purpose of piano music being in anything besides C major or A minor, if the piano is used as a solo instrument... (baring special cases like Bach's Well-Tempered Clavier)... Thanks.

Well, no two keys sound the same. Listen to Beethoven's 5th Symphony and then jump on your piano and play:

C# C# C# A / B B B Ab...

Then follow it by playing

G G G Eb / F F F D

If the sound the same to you then you need your ears checked lol

P.S. Equal temperament has actually ruined a lot of harmony.

In Beethoven's day, an F# an a Gb chord, written for strings, would have sounded different.

The F# should be one comma higher than the Gb, unless the F# is part of a Descending scale or the Gb is part of an Ascending.