When a user has unmatched you, normally it is not possible to access their profile any more. When Unmatched Profile Access is provided, you are still able to go in and review this user (where reviews available) or report them, for example, if you detected a problem with the profile.
This is a great security feature because scammers often unmatch you when they realise you have detected their scam – in the hopes that they won’t get reported. With Unmatched Profile Access, you can report them anyway, thereby reducing the scammers on an app.