So, do you want to be 100% sure that your domain is online? Backup DNS for your Primary DNS service is a handy addition that will make your DNS network broader. If you use a Backup DNS, you can add multiple nameservers that will be authoritative for your domain and answer queries.
Backup DNS service (Secondary DNS) is an additional DNS service that you can get from another DNS provider, different from your primary, with the goal to add extra redundancy. You can use extra nameservers as authoritative, and they can answer queries too.