Multitenant applications in what is now called Entra ID, but what I am sure I will continue to call Azure AD (AAD) for some time, are a way for developers to create Azure AD-backed applications that are managed in the developer tenant but allow users to log in and manage users from their tenant. They are a very common way for SaaS developers to manage application authentication whilst leaving the user management to the client.
If you are using Azure DevOps to deploy resources to Azure then you probably know the pain of using service principles for your service connection. Using a service principle involves storing the credentials for this in Azure DevOps. Assuming you’re following good security principles and having an expiry on the service principle secret you’re probably going to forget to renew it at least once, and your deployments will grind to a halt.