Rubens My easy version: if it's too dry the wood will crack. If it's too wet the strings will rust.
And even before the strings rust, my understanding is because the wood shrinks when it's dry, and swells when it's wet, over time this will cause the wood to warp, it will cause joints to loosen, and it will cause glue to fail and lamination to separate. And yes, enough cycles and the wood will crack when dry. This shrink/swell "breathing" is the slow death that is the fate of every wooden acoustic piano.
Controlling the humidity minimizes this movement, and prolongs the steady-state life of the instrument!