When you fight using a skill, it raise your attributes. The rate depend on the skill you are using:
Melee: Great incrase in strenght, moderate increase in Dexterity, low increase in intelligence.
Missile: Great increase in dext, moderate increase in Strenght, low increase in Int
Magic: Great increase in Intelligence, low increase in Dext, low increase in Strenght.
Build a man a fire, and you keep him warm for a day. Set a man on fire, and you keep him warm for the rest of his life.