Why to learn docker?

If you create applications, it's good to know Docker so you can create a standardized build environment. You never really want your application release builds to come off a developer system, and it's even better if the standard build system can be defined in software.

If you want to run a specific version of a database on your development system, then Docker can do that pretty trivially as well.

If you have problems with version conflicts between various installed environments (Python 2 vs 3, various gcc[lib] versions, other tools installed globally, etc.) then Docker can be useful to set up the perfect environment to run a picky tool, and once it's working you can share that environment with a coworker and know that it will just work.

Docker is a tool. You should learn it if it will help you do your job. The above examples are not exhaustive. Any time that it would be good to have a standardized environment on your development system, for testing or sharing or running specific software, Docker would be useful.