What Deunis said, except it is _not only_ an anti-drill plate. The fact that it rotates freely makes it anti-drill on top.

Originally, these were an upgrade for existing lock cylinders which didn't have built-in protection against [being pulled out](https://youtu.be/bi1lMG3Lfmc?t=49), which became a popular method (_the_ most popular?) of breaking in the 80s. Locks at that time didn't have the rotating bolt sufficently reinforced, so you would basically just sheer it off with brute force. Fast, easy, not much noise, and upon casual inspection people who are passing by do not even notices, once you are inside and have closed the lockless door again (really, who looks at the keyhole!).

Modern quality locks (albeing not the cheap ones!) have the bolt made of massive, high quality steel to prevent that, or have other measures such as a thick end on one side, so they can only be inserted and taken out from one side (well, without smashing the whole door to smithereens). For example, Keso locks with an internal knob have the knob explicitly manufactured in such a way. You would have to punch a fist-sized hole into the door to open it that way.

I don't know the English term, in German they're called "Ziehschutzrosette" (translates approx. to "pull-protection collar", though I'll admit that sounds quite clumsy).

Which makes drilling the lock the _second_ most lucrative option.

It is true that you can still insert a small drill in the slit, but that is pretty useless. Though you'll likely want to start small because it's really no fun drilling in steel with a large caliber, you will need to use increasingly larger drills up to about 10mm to do the job. You can't do that, at least not easily, because the two plates which are made from hardened steel will prevent the drill from entering. And, as they are rotating, they'll turn away as the drill tries to drill through them. So that's drill protection as an extra feature.