Chassis grounding short circuit
so how does the earth ground help on the enclosure ?


An earth grounded enclosure is there to protect you from an internal electrical short inside the enclosure.

If the enclosure has a proper Earth Ground connection to mains, then an internal short to a metallic chassis will be safely shunted away to earth ground.  One should be able to still touch the metal case in such circumstances and not get killed.

The added benefit (and in low voltage DC applications, many times the main benefit), is EMI shielding; protecting outside circuits from the noisy radiating internals, and protecting your internal circuits from noisy radiating external sources.

As an aside, many power supplies allow you to connect either the DC+ or DC- output terminals to the chassis (earth) ground.  This allows you to reference the output to chassis.  In effect, biasing DC- output to the same voltage as the chassis, or alternatively, connecting the DC+ output terminal to chassis (producing a negative biased supply rail).  This depends on the capability and design of your power supply.  Most commonly, one would connect the DC- output terminal to chassis ground for a positive output supply.  This is not exclusively for safety reasons, but for electrical reasons (learn about common-mode signals).

Good Luck!