Secondary emails are useful in case you forget your Okta password.
If your helpdesk administrator allows you to receive 'Forgotten Password' help, then you can request a password help link to be sent to your email address. However, if you're accessing your company's email through Okta, you won't be able to access the email that was sent unless you have provided Okta with a secondary email address. In that case, Okta will also send the forgotten password email to your secondary email address — making it easy to access and reset your password.