Imagine this:

- you want to provide LN/ecash services to your family and friends
-you don't want to know who is doing what payments, to offer them better privacy
- You end up running an ecash mint
- you don't want any rando to be able to use/abuse your mint
- you need auth (preferably blinded)

Also know this:
- It's an optional spec
- No one is forced to add auth to their mint
- you are not forced to use a mint that implements auth

Auth has a lot of drawbacks for ecash mints, but in some cases it is an absolute requirement