[Elecraft] OT: Dual Range Bench Supply Use

[Bill Acito W1PA]

I picked up a used HP variable bench supply 
(6204B DC, 0-40V at 0.3A, 0-20V at 0.6A) 
for QRP supply and test (no docs).

Question for any bench techs out there....

There is a switch on the front for the two ranges, and 
the supply has three output plugs: +, - and GND

Is it correct that I should use the + and - for 40v, and
the + and GND (w/ the GND shorted to -) for 20V, or the same
+ and -?

When is GND used? For, say, Op-amps that have + and - 
supplies relative to GND?

Bill W1PA
(embarrassed that he does not know this off the top of his head)