all that rune does is increase EXP, it's hardly necessary especially since there are other runes exactly like it that increase EXP, just not by that much- and they're all stackable.
the souls games in general have been full of obscure hidden things and game mechanics, that's just the way they are. Demon's souls is probably the worst about this since the game doesn't explain how character or world tendency work at ALL and those are kind of critical. Dark Souls is full of busted covenants that also have no explanation as to how they work, and even getting to the DLC that you paid for is an exercise in frustration.
That's just the way Jrpgs are, even FF. Try getting the excalibur II in FF9 without a strategy guide. literally impossible. not necessary to complete them, but there for completionists.