Why is it useful? Ability to add One to many relationship fields on Cards really helps to view the field value at glance rather than to open a record to view field values.
Who would benefits from it? Anyone who uses board.
How should it work? Its similar way how other custom fields work, we were able to drag and drop at Customize record cards to show on boards
Thank you for reaching out. A one-many relationship field is typically a custom table with several columns, and it is displayed in its own tab on the feature details page. I don't believe there would be a concise way to display it on a feature card. Is your use case different, or perhaps you are thinking of a different field type?