Overview: Application Registrations are primarily used to allow access to a system on a user's behalf.
The best example is MS Graph, where you register an app to work on behalf of the current user. So, if I need access to Outlook or a user's profile, I register an app, and the user must approve the request for these permissions when they access the app (assuming a code flow). Some access is granted automatically, while more powerful access requires an administrator's consent/approval.
