As simple as MJD explains but remember the earths have to have separate terminals even in the DB, the computers use the earth to dump electrical noise, if the earth path is ever broken then any metalwork on the computer can have a floating voltage that can be dangerous... this is why its called a functional earth... the more computers plugged in the higher the current flow .. Ive seen offices have 30+ amps flowing down the main earth so imagine breaking that ...every computer would be floating a voltage with a large current waiting to ground down possibly through an unsuspecting user. Its also not good for the electronics if they can't effectively sump the noise down the earth.