@Ann Naccarato Welcome to Microsoft Q & A Community Forum. Run As accounts in Azure Automation provide authentication for managing resources on Azure. When RunAsAccount is created , the following steps are performed.
As mentioned in above screenshot, a service principal is created and Azure service principal is a security identity used by user-created apps, services, and automation tools to access specific Azure resources. Think of it as a 'user identity' (login and password or certificate) with a specific role, and tightly controlled permissions to access your resources. It only needs to be able to do specific things, unlike a general user identity. It improves security if you only grant it the minimum permissions level needed to perform its management tasks.
Whenever you logon to perform tasks, you either need to have login credentials , it can be either login password or certificate. In short, the certificate is used for authentication purposes. If its not renewed, the automation jobs that you have created will be errored out.