As well as being able to enhance armors at the Fusion Master, you are also able to combine two sets to, hopefully, create a completely new set of armor. This is a very costly procedure, requiring 25,000 Gold and 1 Fusion Crystal. You can see a list of the possible armor types to combine and the end result of doing so, by visiting the Armor Lists section of this guide.
Generally, the rarer a set of armor is, the better statistics it will have, so, if you have the money and Fusion Crystals available, it is worth paying a visit to the Fusion Master to enhance your knights.
The Fusion Master is expensive but is an easy way to get rare armor