[aytekin]

One possible use case: Let's say you are building a new app and you don't know if it will be successful. At this point you are supposed to be working on the core. You are supposed to be talking with the users as soon as possible. You are not supposed to spend your time on things that don't matter such as registration, email delivery, forgot password.

If the app becomes successful you can always implement these things in house later.

[adrinavarro]

Right. So instead of working in creating a user system, you integrate your core to another system.

Or you could, well, let's say, use a library or framework and spare you the work of integration too!

[camus]

Please , in every languages there are a tons of libraries dealing with these stuffs, Starter projects with these functionalities ...

    At this point you are supposed to be working on the core

[UK-AL]

Invoicing, Price Plans and Payment. These are not normally built in. Basic user management is, but not those things.