Testing is a good learning tool, compared to just reread stuff. This is because actively recall things helps to consolidate your memory better, and testing also acts as a feedback loop for you to find out the gap of your understanding.
I think job interview can play the same role as our career development as well.
Regardless of your satisfaction of your current job, attending interview will helps you to further improve your skill in your current job.
First, interview exposes you to the actual requirements of the job market, and allows you to gauge if your skillsets are still relevant. If your skillsets are outdated, it would motivates you to improve yourself. If you're still very marketable, at least you will have better sleep at nights, knowing that even if something bad happens to your company, it's not hard for you to get a new job.
Second, interview encourages you to learn technologies that you're not familiar with. Generally, after you've stayed long enough in a company, you'll built a reputation of good at something and not at something else. The reputation system is good from the team's efficiency perspective, because when a particular problem happens, people knows who to seek advice to fix it. The other side of the coin is that slowly you'll be always fixing same kind of problem. From psychological perspective as well, doing what you're already good at is enjoyable, this will also drives you to seek tasks that you're already good at and avoid those that you need improvements on. However, if this trend continues on you'll no longer growing your knowledge. For instance, recently to complete an interview task, I've learnt Docker and Kubernetes, which is more DevOps-related and I won't have opportunity to learn in my daily jobs.
Of course, completing interview assessment and attending interview takes time and energy, so you shouldn't do it all the time. However, I believe if you stays on your current job for more than a year, it's not a bad things to do to keep yourself relevant.