It is the gateway to your computer.
They can view, edit, copy anything in there once in control. They can send MKultra subliminal stuff to slowly depress you, control the advertising you see, such as suicide helplines.
They get all your email, they know your friends, what you do, where you hang out, everything there is to know about you can be learned from that little number. This is why they are never visible, anywhere on the internet!