what it the utility of using add_action with a custom tag to call a custom function?
You can start to have some features easy to add in place creating some kind of framework, this way the code is reusable..
For example our Admin has a toolbar and if we include a new php in functions.php we’re adding more features in the toolbar without modifying in the original admin html
I use do_action() on the framework, and add_action() on the (framework’s) modules/extensions. This way you have a modular framework, easily extendible.