You can keep a notebook for a myriad of reasons, ranging from keeping a diary or journal, writing down ideas, keeping recipes, business ideas or strategies, leesons you have learned in your daily life.
The possibilities are endless.
The big mistake is not referring back to your notebook on a repeated, consistent basis.
You can find real gold there.
But it is easy to make the mistake of making an observation or life lesson and never seeing it again.
Making or keeping a notebook is only one half of the equation.
The other is to refer back and check it, often.