Include Drupal modules inside composer.json file
There are general two kind of dependencies you can make in a composer file.


Other Drupal modules
PHP/JS/FOO libraries


If your module depends on a specific library, you should do like the address module and make sure that the library is present when it's being installed, so you can prevent installation if the library is not available. It's a clever way of making sure you don't install the module in a broken setup, which could cause fatal errors etc.

IF you simply depend on other drupal modules, you should also add the dependency in the info file which should be enough.