It's old school "game balance" summed up in five lines. You don't need to understand why it works (though it doesn't hurt). Just follow the rules.
Every time I think the old school D&D design space is full up, I find a new wrinkle. This parsimony of design is one of the things that drew me back to paper and pencil roleplaying, and it's something I look for in every new release, and in my own writing.
What works at the table?