The reviewer role in the App Store is linked to the snap review process that a company may choose to implement. Reviewers approve software changes made to snaps before they can be published to the Store, if the administrator has enabled the requirement for reviews in the Store.
Another important aspect of the reviewer role is that someone this role can trigger auto-connection of any of the “self-serve” snapd interfaces using the store’s snap dashboard page (I’m not sure this is the correct terminology) per:
Note - the reviewer can only do this if they are not the publisher or collaborator of the snap in question.