You don't have to include a secondary email address but it is recommended.
In case you forget your username or password, instructions on how to recover those items will be sent to both the primary and secondary email addresses listed on the account.
For this reason, it is recommended that you use an email address that is not using a domain that you have through us (such as Gmail, Outlook, iCloud, etc. addresses).
In case something affects the services for your domain, you would still be able to receive notices and account recovery messages.