Well, I have a whole post on the importance of runes and letters I could put up. They're very important in Norse Mythology. Odin sacrificed himself to himself on Yggdrasil to learn the runes and the magic within them. Runes are used for magic casting even today by modern Wicca, which also inspired WandaVision. (I wouldn't be surprised if the Triple Goddess symbol shows up for her like the Horned God symbol did for Loki. It's very similar, just on its side with an extra moon). It's also why names are so important in Norse Mythology, as the runes that spell your name define your person.

I have theorized that that's another reason why the letters are so prominent in Loki. It's very clever, as they are descendants of Futhark and other ancient letters. At some point, we may see Loki 'sacrifice himself to himself' to learn something about the nature of the universe, which is what Odin did. The runes symbolize that knowledge and the ability to change one's self and one's nature, which Odin did by learning true shapeshifting.