So that when you send a mailing you can target people who have not given a gift aid declaration to give one. Also helps to make sure that those you do have a declaration can update their details. At the moment you have to export all Declarations to find the most recent one. You can currently do this with Actions i.e. export in data order, choose descending or ascending and then choose to only export one action.
Crucially it would be good to have this on a GIFT export so that gift letters can be built with rules that can pull in if a supporter has a declaration, a no declaration or no declaration at all. This is not currently possible for records with multiple declarations