Could someone explain the purpose of backflows and why some houses have them and others don’t? I suppose they keep contaminated water from making its way back into the water supply but then why are they not found on every property?

Are you talking about the anti siphon device or the municipal backflow system?
Anti siphon devices became a requirement a while back not sure what year exactly.
Not all homes have them because people remove them and some homes were built before the requirement. There are no anti siphon police just like there are no TPR police.
If you remove the anti siphon device and your house water gets contaminated it’s only you and your family that gets a bad taste. The municipal system is regulated by the utility dept. If the home has a private well That’s a whole different story.

I’m talking about the backflows that are on the lawn near the sidewalk most times. It seems like many houses have them, many dont. I have heard they are somehow required if you have an irrigation system. Confused! Also, I updated the broken link.

…hose-bib vacuum breakers have been a required by the Standard Plumbing Code since 1963. Inspections 101…

I don’t give a crap about a 1963 hose bib code. What matters is what applies now. Hose bibs are still absent on many homes so why don’t you just provide an answer to the OP?

What…you mean they didn’t teach you this in the MSFH program? For the protection of the potable water supply both in your home and the water main. Not “anti siphon”, not “backflow”…Correctly called and addressed by the PROFESSIONS and code as “atmospheric vacuum breakers”. Required since 1962. Purpose is to prevent cross contamination…“life safety and health hazard”, kind of like GFCI, 16 CFR 1201, etc…

OP is probably not talking about hose bibs anyway. Most backflow preventers would be associated with larger water supplies like on multifamily buildings or newer homes in some areas. Anti-siphon devices are always required on irrigation systems connected to potable water supplies.

Listen up TOOL. Was never part of that program, you are side stepping the whole question once again. I really hope to meet you one day.

It is typical that new SFR have backflows set on the water meter (house side). Mandated on commercial for decades. Older residences will not have these until their water meters are changed for some reason.