*throws not-a-boner confetti*
It’s not specified anywhere that they do, but a lot of players draw/write them! I’ve seen a few reasons for them: magical protection, runeblade connection, necromancy/animation, power activation (quick spells, or an interpretation of the rune mechanic), power enhancement, decoration, the list goes on.
Oliver’s runes are… kind of a combination? They correspond with his runeblade, so as it empowers from harvesting souls, that power is transferred to his body, which keeps him mobile and enables him to regenerate flesh wounds (but never past the point he was risen; his face, for example, can not regenerate). They’re also what activate his powers— he draws energy from the blade, and then his body is the channeling conduit for whatever magic he needs, with the runes acting as a sort of permanent incantation.
I’m suggesting this as an open one to followers, too! What do your bodily runes do, if you have them? Help an anon!
Tevruden’s runes both enhance the innate connection he has to his runeblade and connect him with his armor, they generally end on him wherever his armor glows (though this only makes sense when i have him in the variants of the blue starter gear.)
The runes themselves weren’t necessary for his reanimation. He was intact enough, having died only a few hours beforehand, that he could be raised with a bit of minor fixup (not that the necromancer only made minor adjustments.) The runes were then given to him after he proved himself as an initiate.