That C-chord on measure 6 of the system, from Grieg Piano Sonata, Op. 7, IV. Finale - Molto Allegro:
Why couldn't they just mark that D7 chord with a p? That G chord is marked pp.
Once you press a key on a piano and hold it, you cannot modify the dynamics. It will just decay by itself after a while. Why do some engravings have a crescendo or decrescendo over the full length of a single half or whole note (usually a chord)? There are not shorter notes in different layers that you could progressively play louder or softer while you hold the long note. It is not a trill or tremolo.
I just ignore these phantom crescendos and decrescendos when I see them in piano music. I can understand if you're playing woodwind, brass, or organ because it's physically possible.