Watch a video of any elite athlete or performer before a big game or show, and you will likely see one thing: their pre-performance habits, the things that they do every single time in exactly the same way.
This is because habits are everything. Not just for game-day, and not just for elite performers. For normal people like you and I, for raising our children, for being happy in our relationships, for being happy as individuals.
Our routines and habits allow us to access a part of our brain that runs on relatively little gas. The newer (in evolutionary terms) part of your brain—your smarty-pants pre-frontal cortex, the area that sets you apart from the family dog,—works pretty well, of course. But it requires effort and willpower to make it tick. The more you use it throughout a day, the less reliable it becomes. Low blood sugar? Your decision-making will falter, whether you realize it or not.