Regarding the specification on destinations to output airbyte state messages: https://docs.airbyte.com/understanding-airbyte/airbyte-specification#write

The destination connector should only output state messages if they were previously received as input on stdin. Outputting a state message indicates that all records which came before it have been successfully written to the destination.

Does Airbyte do anything with the state messages outputted by the destination, or is it purely for surfacing info to the user?

The state output from the destination is used as the checkpoint if you’re using source APIs