Regularly customers ask "why should I raise ADDS functional level"?
Here are the anwsers
As it regularly happens when you stop a test platform for a long time, computer accounts password expire and Active Directory controllers do not replicate anymore (well known item since Windows 2000 Active Directory).
When you try to manually replicate using dssite.msc, you get the error message "2148074274 The target principal name is incorrect".
When a user opens an Active Directory session with his smartcard, it happens that some applications do not support smartcard logon, but needs classical user/password login.