 At least some, perhaps most, of the value of believing truly can be accounted for instrumentally. Desires mesh with beliefs to produce actions that will best achieve what is desired. True beliefs will generally do a better job of this than false beliefs. If you are thirsty and you believe the glass in front of you contains safe drinking water rather than lethal poison then you will be motivated to drink. And if you do drink, you will be better off if the belief you acted on was true (you quench your thirst) rather than false (you end up dead).