Strapi, --dev flag, problems understanding how to create a project with and without it. How to create a dev project?

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How to create a --dev project correctly when there is a project created without this flag?
I understand that the --dev mode allows us to make changes to the admin panel, but you have to create a project with a different name, for example strapi new my-app --dev . What for? Well, it's not entirely clear how this will relate to another project, but to the one that was created without --dev .
Help me understand, I'm confused .