MoBo insulated from ground?

I noticed while removing my no good,to be replaced, Epox Main board from the tower, that the threaded bosses? in the chassis have a plastic insulator shaped like a top hat with a small bolt size hole in the center of the top pushed down over them. each of the small screws that fasten the board down has a close fitting fiber washer. It looks like they have insulated the board from chassis ground. Is this so? If so why? Just curious. I don`t see anywhere where the board is intentionally grounded, or earthed, as they say overseas.

 

It gives "Cleaner" power to the MOBO if the PS neutral is isolated from the cabinet ground.

The cabinet is grounded to the incoming a/c supply ground conductor (per NEC 250-45) and that ground is often very "Noisy. "

Isolated ground systems are very common in critical applications. The POS equipment found in most retail stores, as well as banks and other places that handle critical ($ related) data, have recognized that point for years and do not tie the system ground or neutral to the cabinet ground except when an additional isolated ground conductor has been supplied (per NEC 250-74, exception #4) that doesn't bond to any intermediate junction boxes but only to the main building ground.

 

Thanks