Paraphrased from minutes of PrivacyCG meeting on Dec 10 2020, summarizing @johnwilander's comment:

It would be nice if the SameParty attribute could convey contextual information from the site developer to the user, e.g. what functionality the cross-site cookie usage would enable. This information could be displayed to the user in a permission prompt (along with how long the cookie would last), which the user would have the option of accepting or not. If accepted, the user agent would store the cookie and send it according to SameParty semantics.

(@johnwilander, please clarify/correct this if I've misinterpreted your comment! Thanks.)