Your Primary email address is where you will get the important information about your account. The Notification Email address you see is a carry over for when devices did not use Two Factor Authentication. Before 2FA, you used two-step verification where you had the option to use a Rescue Email Address in case you lose access to your account and account related information was sent there as well. After you updated to 2FA, the Rescue Email Address went away and was simply named a Notification Email address, losing the function of being able to reset your security questions.
About your Apple Account email addresses - Apple Support
I suspect you are a long time user who has had their Apple Account for a while. You now have the option to edit the Notification Email since it was migrated from the previous Rescue Email, but if you delete it, then you will not be given the option to add it again. The information sent to it is just a duplicate of what you are receiving on your primary address with 2FA and cannot be used to recover your account the way it was used as a Rescue Email Address using two-step verification. It would not make sense to have the same email address in both places. My recommendation would be to remove it, but maybe someone else will jump in for some reason why they like to have the duplicate emails.
Instead of relying on the Rescue email, with 2FA you use a Trusted Device and Trusted Phone Numbers to access your account if you forget the password. In addition you can add a Recovery Contact to assist with Account Recovery.
Set up an account recovery contact - Apple Support