Talk:Supernatural Abilities/@comment-29735342-20161011074507/@comment-16533050-20161011074953

I would surmise that they are carved and treated as offerings to the Outsider. A good deal of the runes are found on Outsider shrines. When enough of these are created and given over to him, the power of the one making the offering increases.

However, that's just head-canon of mine... so take it with a grain of salt.