Your beliefs are how you model reality. Reality consists of objective rules of cause and effect. A belief is an estimate of how some piece of reality works—an explicit but putative mapping between cause and effect. Each belief is a prediction of the outcomes that will result from certain actions.
Beliefs are the key to knowing how to obtain what you want in life. They are recipes for how to obtain specific outcomes. The more of reality you have mapped through beliefs, and the more accurate those beliefs are, the closer you will be to knowing how to obtain what you want most in life.
Beliefs are also crucial for determining what you want in life. The more you understand what is possible and what it costs, the more informed your appraisals of value will be. To the extent they are complete and correct, your beliefs can be used to prioritize potential action and bring anticipated joy forward to the present.