Instead of item type being arbitrary "levels of items" such as "Short swords are weaker version of long sword", item roles create a distinct type template that provide benefits for using both item types - usually by adding specific bonuses for using the item.
Example: short swords have a bonus for +Speed, Long swords have a +Range bonus, and Bastard Swords add +Stun duration, as basic magic property(Automagic), and "Sword" class weapons have the same damage output.