If the Wealth-By-Level is an indication of the value of Magic Items for Player Characters to use in combat, does the WBL for NPCs have the same purpose, or is it there for some other purpose (to provide treasure for PCs, say).
WBL for NPCs
The WBL for NPCs is largely to give NPCs the kind of adventuring gear expected (or required) by adventurers to present a challenge (either to PCs, or as allies when fighting other NPCs). A Wizard without a spellbook, or a Fighter without arms and armour are easy meat and do not correctly reflect their Challenge Rating.
The WBL for NPCs is not intended to override the normal treasure guidelines in the WBL entry in terms of loot for encounters/expected WBL, and NPCs receive less WBL than PCs partially due to this concern.