Why does odin visit mimir?

Mímisbrunnr, a well of knowledge, which Odin gave up his eye to in payment for a drink. Mímisbrunnr is guarded by Mímir, who was a friend of Odin. After Mímir got decapitated during the Æsir-Vanir war, Odin embalmed & enchanted his head so Mímir could still talk and started carrying it around with him.