The purpose is well meant generally, it prevents contaminated water entering the fresh water supply, I remove them myself and use a check valve after the pump with a pressure reducer, this keeps water after the tank from moving towards the tank rapidly.
Its over kill if you ask me any of these strategies and is based on the premise you will not maintain the system or understand it so you must be protected from yourself.
If the shower head laid on the floor too long and if the gray water system bacteria
made its way to the hand shower nozzle the vacuum breaker will block the path for bacteria if that cheap plastic Chi---e made crap works...However as cheap as its made
its 50/50 it does anything.
Its a device a manufacturer must use
to get the required safety ratings, they have no choice...Trade associations and health departments work in concert with insurance data bases to coordinate strategies to mitigate risks.
Once its installed, then its up to you to maintain it and on your head if removed...if it fails prove it? If you prove anything try to collect damages? When your 90 years old the check arbitrated to 100th the actual damages might arrive on a leap year.
I would do as I please within reasonable boundaries... I sanitize my tanks before every camp out as good stewardship of my rig.
Happy Camping, Safe Trails.