In order for us to develop more and better app features, we need a way to store your data in a reliable manner. That's why we ask you to sign up for an account in the app.
For example, any in-app purchases you might do, will be safely stored in your account. This means that if you would ever switch phones, for example from Apple to Android, you will still be able to enjoy the Premium features which you already paid for.
Additionally, Premium users can record their calls and those recordings get stored in your account so you'll never loose them.