For the most part there isn't any need to, with the exception of things explicitly meant to be shared. So if you are a large company and want a bunch of custom auth logic or something to be shared among many different teams running their own Django service, then you might want to build that as an app. Or else if you were building something as an open source project you might want to do the same.

In terms of separating apps by functionality, I think the main reason many people do it is because they are under the mistaken impression that putting things into different apps will make them magically re-usable. That's not to say you shouldn't do it, only that I think most people do it for the wrong reasons.

In general I would say don't bother splitting up your project into different apps by functionality until you have too many models to comfortably fit into one file. That might not be until 20KLOC. For apps that are less than 10KLOC, I think in most cases the overhead probably outweighs the benefits.

There isn't really any hard rule one how to do this, but you might do something like:

- Account management app for creating and deleting users, getting user profiles, password resets, etc.

- Apps for integrations with third-party APIs, e.g. Duns or Mailgun.

- Apps for different front doors, e.g. if you make a Facebook chat bot that's powered by the same tech that powers your normal website.

And then after that you can possibly split up your business logic of your core website into different logical components, but I would do that last. By the time you get there you should start to have a feel for how to do this.

The one place I use it for our own startup is that we recently built a couple different front doors for our site, and each one accepts data in a slightly different format and has different authentication requirements. I put each of these in their own apps, but everything else is just in one main app.