Guthix created runes in the first age, here on Ashenfall. He found a way of capturing the ambient wild anima, of the continent, in the floating rune essence around the region. He would then give these runes to his druids, teaching them the fundamentals of magic.
Fire runes contain enough magical fire to light a campfire. Battlemages prize them for the ability to lob that small fire at an enemy instead. Either way, they’re a very handy resource.