Why was the battle of the Monitor and Merrimack important?

They were the first heavily armoured warships. After news of the battle reached Britain we immediately made plans to rebuild the navy in the new armoured form.

They were not the first ironclads actually. Britain and France already had multiple ironclads in their navy. The real significance was that it was the first actual engagement of an ironclad in battle. It brought about the age of iron ships because it made wood obselete. It also kept the Union Blockade standing.