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Earth is their because some people use it to stop their "0v" rail floating up to dangerous potentials...it may get "induced" up.
But generally its because its the norm in a system to just make sure every 0v in a system is , if poss, connected to earth.
This is done in such routine, that people forget why....but generally its safety.
True story - I was there....
A newly built factory in South Wales called me in to investigate grounding problems. The facility was defence approved and making various military equipment and secure communication products including some highly classified technology. The builing was basically a huge Faraday cage with a copper mesh floor (~20,000 square feet of it!) but they were seeing problems of electrical leakage to the incoming power lines and couldn't figure out what was wrong. When I can only see a small part of the installation at any time it was difficult to figure out what and why there appeared to be random Earth currents flowing around virtually everywhere in the building. I eventually got the (top secret) plans for the building construction and discovered there was a single connection point for the Faraday cage. It was well away from production areas where the problems were being reported so it's no wonder it was difficult to find. Lifting an inspection cover in the floor revealed a large braided cable, coiled up and secured with cable ties - but not connected to anything!
The building contractors didn't know what to do with it so they just made it look tidy and covered it over. Connecting it fixed the problem.
So it isn't only PSUs that have grounding issues, it can be whole factories!
Brian.