Your beliefs will help you write and update every section of your life plan to maximize your joy: they will help you specify needed qualities, list and rank the relevant values, suggest necessary goals to achieve, and enumerate required tasks.
Your beliefs also serve as a sort of interface between the why and what sections of your life plan through the links provided in the cause and effect mappings they describe.
Finally, your beliefs provide the mechanism through which the lessons you learn through living the contents of your life plan can be propagated up and down between successive sections of the plan.
In essence, beliefs help you figure out what you want, develop a plan to obtain it, and update your course as you integrate necessary changes.