When a user installs your app, they will need to have an SDK-associated account. You create that account with this call, and receive the user credentials in return.
Note that the API does not qualify the user fields at all, and in fact, all fields can be empty. The only user-specific data is then the device token. If you have certain requirements for some fields (first name must be at least two letters, email domain must be mycompany.com, etc.) do an input validation in your app before sending these fields to the API.