Most people/homeowners do not know if they have one installed or not and what the real purpose is for.
The purpose of all of this is to prevent potential cross contamination or cross connection. This is a connection that has the potential physical connection between the water supply and any non-potable fluid, solid or gas.
The potential with a hose bibb is that if you had the hose connected and something happened that created back siphonage or backflow you could potentially suck contaminated fluids (solids or gases) back into the water supply.
What could cause this? There are a few things that can cause this, a break in the water service, a sudden drop in pressure from high water withdrawal (like from fire fighting activities), when the municipality performs flushing of mains and/or reduced supply pressure on the suction side of a booster pump.
Now granted not all of these are going to be common, but protected the water supply is a growing concern in many municipalities. As for most plumbing codes it aimed at health and safety. Keep the bad stuff in its place and away from what we will drink and cook with. If you have any questions regarding your homes plumbing systems call Roberts Plumbing, Inc.