I've been trying to understand these two bad boys for a while now with no reasonable results.

You can quite easily use OCOL to get your masks for objects (keying/swapping RGB channels,...) and you can do the same using Baking Group ID and Layer ID but that feels like weird answer given that the colors are quite strange and random.
Why are there these two passes? Why the word "baking"? Why do they work the same way? Can I use the ID meta information (if there is any) to separate the objects in Nuke for example?