Yes, even if it’s on a sidewalk, a napkin, or the back of a sales receipt any writing a writer will do is important. Any writing will improve a writer’s skills, could lead to new ideas, and is better than doing nothing and yielding to procrastination and writer’s block.
It’s sort of like going to work. If you sit in the cafeteria all day you’re accomplishing nothing, your productivity is nil or close to it, and you’re not earning your pay.
So, if you’re a writer and you haven’t written anything all day, go get a piece of chalk, an envelope, a sales receipt, anything and write. First try for 100 words, then 500, a thousand, and keep writing. Right now, it makes no difference if it’s publishable, if anyone’s going to like it, you’re writing and your writing will eventually lead to all that stuff as long as you keep writing.