Maybe this is a stupid question, but here goes:
I just got a piezo buzzer and on the top there is a sticker that says "Remove After Washing".

My question is why would I ever want to wash an electronic part? I have no idea. Is there some manufacturing step where this makes sense?

The industrial PCB assembly process usually leaves residues — mostly soldering flux — on the circuit board. One step in the process is to wash the board (by dipping or spraying) with a solvent to remove those residues for long-term reliability and for the sake of appearance.

Some devices (such as sound or pressure transducers) have openings for their functioning, and their performance would be adversely affected if the solvent or the residues got washed into the opening and lodged there. Therefore, such devices often have a sticker that covers the opening(s) that should not be removed until after the washing.

Removing the stickers adds an extra step to the process, so for really high-volume manufacturing, it is often worthwhile to select parts that are declared "washable" to begin with.
