God's people need God's power to fulfill God's purpose.
"And what does your anxiety do? It does not empty tomorrow of its sorrows; but, oh, it empties today of its strength. It does not make you escape the evil, it makes you unfit to cope with it when it comes. It does not bless tomorrow, and it robs today. For every day has its own burden. Sufficient for each day is the evil which properly belongs to it. Do not add tomorrow's to today's. Do not drag the future into the present. The present has enough to do with its own proper concerns." - Alexander Maclaren
What is the point of fasting? It reminds us of dependence. It teaches us self-denial. It's part of a lifestyle of devotion. It makes us patient with delay.