Hi there,
If you don't read the documentation when initially starting with Django it gets overwhelming.
So it is recommended to modularize your apps in that your app should do one thing. That's to say, if you wanted to have user authentication and blog with ability for users to add comments you could have separate apps for that logic. Eg, accounts app for user registration and authentication, then the blog app for the blogs and comments.
Note that the blog app can further be divided into the blog and comments apps.
Depending on the size of the app, the developer can decide on whether to create more apps or just do one monolithic app.
Rule of thumb, if the app can stand on it's on, it's better to create than add to an existing one.
Hope this helps.
Best Regards,
Albert