Item Customization
As players progress through the game they need to change their equipment from time to time. This is because all equipment items have minimum skill thresholds below which they are not terribly effective and maximum skill thresholds beyond which they hamper the player's actions.
While basic items of all levels are available for sale from NPCs, crafters are capable of designing and constructing unique items that are far more interesting for given situations. Non-crafter players can progress through the entire game without ever interacting with a crafter if they so choose. This said, crafters are capable of crafting items that are far more interesting than those available from the NPCs.