The documentation for init.m is functional, if sparse. There are no Examples, and there is no Applications section.

The init.m documentation points to Wolfram System Setup under "Related Guides", but that topic apparently doesn't point back to init.m, or at least, not directly.

The "See Also" for init.m contains four items, three of which are not helpful in the least. Those three are wolfram, mathematica, and Mathematica, all of which appear to be largely nonsensical in this context.