The "researcher" user right was originally created to allow researchers the ability to process the history of Wikipedia more effectively. A lot of research that is relevant to Wikimedians requires access to deleted content. For example, building machine learning models to catch spammers, track copyright violations, and otherwise model bad behavior requires access.
Right now, we work with the WMF Trust and Safety to to help researchers get access to deleted content (in combination with a Non-disclosure agreement and a Memorandum of Understanding), but the user-groups that we can give them are somewhat inappropriate since they allow for much more than access to deleted content. E.g., sysop and staff rights are overkill. This "researcher" user right is a good opportunity to grant temporary and well-scoped access to these researchers.
See the "researchers" user group and some details about its origin: