It has less to do with humid environments and more to do with grounding in general. You can find 3-pin outlets in most bathrooms in Europe, they're typically used for hair dryers and washing machines.

If equipment is designed with sufficient insulation that prevents users from coming in contact with the mains voltage in cause of malfunction, 2 pins are generally enough. Otherwise any electrically conductive parts need to have a connection to ground through the 3rd pin to prevent risk of electric shock.