They're hose bibbs with integeral vacuum breakers, and in a lot of places are required by code. A type of backflow preventer. A big concern for water main contamination (for example) would be filling up a swimming pool full or filling a tank of some sorts with a hose, then having a watermain break, then having the tank or swimming pool water siphon back into the watermain.
That's the kind of things hose bibb vacuum breakers protect against, but they suck untill you get used to them. You always have to get the pressure out of the hose before you turn the tap off, or they spray like crazy.
Hand sprayers are the worst, you have to hold the handle on them down at the same time as you turn off the hose bibb.
If it sprays evey time you turn the water on, I suspect you have a real cheap one, they shouldn't be doing that.
