It’s not an essential. Nothing is exactly essential for a writer, because remember, there are no rules. However, there are things you should try, and then figure out whether they are write for you.
A writer’s notebook is where all you brilliant imagination and creativity is imprinted for the sake of your memory. Notice I say ‘brilliant imagination and creativity’ – as useless as an idea may seem to you, write it down. Your brain may link it to a new character, a new plot, and voila, you have a premise for your next story. I find this one of the hardest things to understand, but as time goes on, it proves more true, as I hope you will realise too.