If you don’t take weakness of will into account when making plans or predictions about the future, you will be wrong.
Corollary: When making plans, it is crucial to take into account the mental / physical state that these plans will place you in. For instance, with respect to plans that might change your objective function, it’s important to make sure that you are changing your objective function in a direction that is good.
Example: You have some project, and you think it’s a really good idea to stay up really late working on it. It’s probably better to sleep for multiple reasons, one of which is that lack of sufficient sleep probably leads to higher levels of akrasia tomorrow.
Remark: Regulating things like food-water, sleep are important for increasing strength.