As a result of frequent loss of water pressure in cities, plumbing codes mandate that every faucet outlet have backflow protection and cite the applicable standards for each type of outlet.
Some years ago, I was on a business trip to Germany and had a room on the top floor of a hotel. In the early morning, I was amazed to hear a loud sucking sound coming from the bath. There was no water pressure at the fixtures and the sucking sound came from a fitting on the wall over the shower.