Right now only the MessageResponse returns the data you pass in with the extra parameter. I use this field to know which session a response is associated with, which I can't do if it's not included in every message type.
The docs make it seem like it would be included in every API response: https://developers.deepgram.com/docs/extra-metadata
But I'm particularly interested in UtteranceEndResponse at the moment.
Extra Metadata allows you to label your requests for the purpose of identification in downstream processing.