The Main Repository private key signs all unsigned content in the Main Repository.
Agents use the public key to verify the repository content that originates from the Main Repository on this ePO - On-prem server. If the content is unsigned, or signed with an unknown repository private key, the downloaded content is considered invalid and deleted.
This key pair is unique to each server installation. However, by exporting and importing keys, you can use the same key pair in a multi-server environment. Doing so ensures that agents can always connect to one of your Main Repositories, even when another repository is down.