Cosmic runes are used in the Magic skill to cast enchantment spells, and the Create Gatestone spell within Daemonheim. If you are a member, Cosmic runes can be crafted at the cosmic altar in Zanaris via the Runecrafting skill. Doing so requires a Runecrafting level of 27 for 8 experience per Pure essence. At level 59, the amount of cosmic runes created per pure essence is doubled.
Cosmic Runes themselves have multiple uses, but mostly for enchantment. Enchanting jewelry, Crossbow Bolts, and Multiple other items is where Cosmic Runes come in handy. In addition to enchanting, Cosmic runes are a Wizard's Staple and always in demand. Crafting Cosmic Runes is profitable.