One of the most important areas to consider when you are a developer is how well you are progressing and what areas you need to improve in. Having a skills matrix allows you to gauge your progress which makes it a lot easier for you to set out a roadmap for areas that you want to gain knowledge in. I think this is also a great way to combat some aspects of imposter syndrome as you can have a visual idea of your overall technical stack as well as knowing what type of level you feel you are at personally. Many companies already use such a format as they like to see how good their team of developers currently are and how capable they are at working on a variety of projects.
The data that you use for your skill matrix is likely to be completely dynamic as you are going to learn new things throughout your career as a programmer. And as this data is dynamic you can visually see how well you are doing and plan ahead for the future. Something like this is key regardless if you are currently looking for work or already in employment. As the industry keeps changing with new advancements in technology and in the tools and programming languages that we use day to day. Keeping your skills fresh and up to date with a structured skill matrix can give you much added confidence in the work that you do.
See the examples below for an example of how that could look.
Skill Matrix Graph
Skill Matrix Table