Why use 2 isolated grounds in Mackie 220W Power Supply?
First we need to realize that this power supply for a large format mixing console, probably with about 40 channels, and 16 mix buses. As a result, there is a need for multiple supply rails. This breakdown may not be exact for the console that goes with the power supply, but in general:


48V is always the phantom power bus
+/- 18V is for all the audio circuitry, and contains a +24dBu signal with 1.2V headroom at each rail. 
12V powers any cooling fans and goose neck lights
5V powers any digital logic for mute groups, scene memory, etc.


The +/-18V rails need to be earth referenced by convention, otherwise ground loops will be a problem when the console is interfaced with other equipment downstream. The 48V bus also interfaces directly with audio circuitry, so must be referenced to the same ground. The 12V and 5V buses are powering electrically noisy things. To keep that noise out of the audio signals, they need to be kept isolated from the audio ground.