I think I need to correct some misunderstandings I have about AEAD. What is associated data used for in authenticated encryption?
What the associated data is used for: Anything you want to use it for.
If you have some data that you want authenticated, but does not need to be confidential, then you use that as "associated data". For instance, consider a packet encryption scheme. You might want the packet payload to be authenticated and encrypted, but the packet header to be authenticated but not encrypted. In that scenario, you could make the packet payload the "data", and make the packet header the "associated data".
AEAD provides a generic primitive, so you can choose which data needs both confidentiality and authentication, vs which data only needs authentication, based upon your needs in any particular situation.