Power supply ground

[t_ryner]

Hello! This is my first post here, and I just finished prototyping a audio amplifier on a breadboard. I plan on powering it with a power supply module I salvaged out of an old DVD player(I checked the power module's outputs with a meter). However on a previous audio amp, I had a problem with it not being grounded properly. I noticed that on most commercial power supplies, the ground planes are split internally, but are linked via a pair of optocouplers. Why is this? why not just bypass the optos and connect to a earth pin?  ( with a simular but smaller power module, it used a chasis ground to the metal case. when connected to power and earthed to the outlet, there was a sudden and violent release of magic smoke    ) Why is this?

[Zom-B]

• Allow multiple power supplies to be put in series (create balanced +/- voltages)
• Inject voltage in the middle of a circuit that's already grounded

[Codebird]

As above, plus:
 - Ground loops and common-mode noise when two such devices are used together. Creates the annoying hum and puts out a ton of unwanted RF.