Just the fact that you are wearing armour of some type does not mean you are instantly protected from some damage. There are many different types and styles of armour, each needing their own skill in use to be effective. Platemails must be worn in order to absorb as many blows as possible. Softer armours, such as leather armour, must deflect blows, since it cannot absorb the damage that a set of platemail can. The more skilled one is in using armour, the more effectively he can protect himself from damage through its use.