Plumbing – attached to this spigot and how to remove

This adapter is attached to an outdoor spigot:

It is near the location of a former hot tub so I believe it has something to do with pressure control. When I attach a hose to it and turn it on if the hose is not flowing (e.g. a closed nozzle at the end) the water leaks (more like squirts, lots of it) from this adapter, it appears to be from the holes and not the connection to the spigot or the hose.

What is this thing? Is there a way to utilize it? I'd like to remove it so I can use this spigot but when I try to unscrew it with a wrench it does not budge – in fact the spigot itself starts to rotate. I wasn't really able to tighten a pipe wrench to the spigot due to the available clearance & pipe width.

There appears to be a rivot on the under side of the adapter but it is difficult to tell, it is somewhat rusted over.

Follow Up

My original concern was water leaking from it when it was used. While a plumber was on site doing a different job I asked him to look at this to see if something was wrong. He said the vacuum breaker wasn't working properly and ended up replacing the entire hose bib (with new vacuum breaker), I presume because he was not able to remove the vacuum breaker as well or it wasn't salvageable. Now no water spews out when it is used.