Early in my career, I experienced the most impostor syndrome when I was left with nothing to do. I would come to work like a dock worker, waiting at my manager's desk, to see if she had any work for me. She would shuffle around her email to find something. Anything! Anything that she could give me to keep me busy. When the task was to adjust some CSS, or change the text on the website, I felt useless.
But it didn't take long until I figured out that down time was my most valuable time. First of all, it meant that we had achieved our goals. We had finished all our tasks on time. Something to celebrate. And it also meant, I had free time. Time I could use to experiment. Time I could use to explore.
This free time would prove valuable in learning new things. I was a front-end programmer, but in my down time, I familiarized myself with C. It allowed me to read the PHP source code and learn to better use the language.
In my down time, I read books like Game Programming Patterns by Bob Nystrom. It allowed me to explore new ideas rarely encountered in the Web programming world. It even helped me when I built applications for embedded devices later in my career. It allowed me to work on writing better documentation. Finding effective ways to describe a program that allow people to use it.
My down time became productive time. So much so that I would never complain when I finished all my other tasks. This time that my employer couldn't justify was when I learned new things that benefited myself and the company. At first, it felt like wasted time, but now I see that it is the experience that made me a competent programmer.