Rewards for crafting the three normal versions include three Keys to the Enchanted Chest. Rewards for crafting the three plus versions of these armors include 5 Dark Prince Keys and the limited edition Night Watcher's Cape.
You need to keep these armors in order to claim your rewards. That means you can't fuse them away nor use them to enhance other armors.
It was previously stated that you also couldn't have your knights wear these armors in order to be eligible for rewards but it's been proven that this is not true. (See screenshots and comments on 'Horseman's Battlegear'.)