re: What is this thing for?

Here is why they are on your spigot...

When you use interchangeable speed bibs, say, for example, you have numerous nozzles, sprinkler heads, etc., you put a quick release bibb/nipple (male) on each attachment or accessory. The opposite (female) on the hose end or spigot end.

This bibb device will stop the water flow if it doesn't have pressure pushing it out (hence the spring), thus cutting the water supply, thus allowing you to change attachments or heads on the fly without having to go all the way back to the spigot to turn the valve all the way off, then change heads, then turn valve on again.