If the SQL Service is configured with a Domain Run-As account you must register the SPN Manually
If the SQL Server Service is running under a Virtual Account(NT Service\MSSQL$<InstanceName>), Network Service, Local System or Local Service you do not need to register the Service Principal Name (SPN) manually.
To find out account what your SQL Server service is running under, open Sql Server Configuration Manager | SQL Server Services or services.msc on the SQL server and take note of the account.
The default SQL instance listens on port 1433.