Depends on the exact type of suppressor, but generally it's to guard against transients. In other words, if there is a sudden high voltage spike on the circuit the suppressor will appear as a dead short to ground preventing the voltage from showing up at the circuit and destroying it instead. A Voltage supressor can be as simple as a properly valued capacitor or an active device such as a diac. Generally they're meant to survive the brief pulse that they gobble up, sometimes they're used to trip fuse.