[MisterWebz]

That same person is also standing at the entrance of almost every other building. To make his job easier he's also given you a unique identifier, so that he can notice when you enter a building he watches. I'm sure you've never noticed him, most people don't.

[Drakim]

And at the end of the day, he goes over hist list of what buildings you went into, to analyze what sort of things you like so he can show up at your door as a door salesman to sell you some product he believes you will be interested in.

[JohnTHaller]

He handed you a unique identifier. You're the one who is keeping it safe and showing it to him every time you see him. Unless you manage your cookies a bit better.