Say you want to easily create a comments app or ratings that you can associate with any other model in your project. Write that app once and use in many places. Or maybe something that sucks to write that may be error-prone, like auth, especially social auth which may change frequently. Maintaining one app and sharing it is way better than everyone trying to keep up with the changes themselves.
Can't you just keep the logic for your comments app in the monoapp and use it with everything?
I understand if you want to distribute your app for others to incorporate, but otherwise it felt like the documentation overemphasizes the "app" aspect. It's a reasonable structure but more of a guideline... unless you want to reuse in other codebases
Or take a look at https://djangopackages.org/ for examples of apps people like to share.