Writers are like other people, except for at least one important difference. Other people have daily thoughts and feelings, notice this sky or that smell, but they don't do much about it.
Not writers. Writers react. And writers need a place to record those reactions. That's what a writer's notebook is for. It gives you a place to write down what makes you angry or sad or amazed, to write down what you noticed and don't want to forget . . . .