If you have a module for things you do on your own, leaving it as only a module and putting it in your LOAD_PATH makes sense. I have a few little things like that.
If you plan on sharing the material with others or, more seriously, using it to support a journal `publication, then it makes a lot of sense to put it in a project or application with the right .toml files. That make the dependencies clear and helps your colleagues/readers/collaborators reproduce results. The escalation from module to application/project almost surely means you have to learn how github supports that stuff, learn enough about pkg to build the .toml files, and put X.jl in the right place.
If your project becomes a bigger deal, say a tool for a class you teach, a group you lead, or a book, then making it a package is a good idea so you can do CI and distribute it more easily.
This is, of course, my opinion as of today. I’ve changed my mind on this a couple times in the past.