The purpose of them, as such, is to provide a source of evocation during ritual. The Keys are invoked because they sound powerful, and because they are productive towards a ritual atmosphere/goals.
As for "why them"...you've surely read the translations.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."