Why is motherboard mounted on a system case

to ground it and protect its components from electrical surges and shocks

Can you provide more information please? Your question doesn't really make sesne.

Read literally, I would answer:

The case in turn provides support for all devices needed for a computer to operate. The motherboard is mounted on a system case to provide a safe and stable platform for all necessary components that connect to the motherboard.

LOL, if you really want to find out why, put it on the floor and turn it and go on a walk for a bit than you'll see what happen... The case is just to give it better cooling and give it space to put your hardware in... Do you want your video card to just fall/comes off and cause your motherboard to die because of electric shock... You should can put or mount it on a custom board for the back ground just so that it holds and leave it there but I warn you, your ganna rage at those wires and other stuff when you want to move it...

EDIT: You need a start-up/reset button! Cant turn it on "If there's" no start-up/reset button... Most recent board usually have start-up/reset button built in...