To ensure an easier admin task and make integration with employee system easier then assigning access rights through the member groups will be great.
Ex in an ART (Project / Planning Level with subfolders) we have maybe 10 Scrum Masters, currently we have to set the Project / planning Level Assignment for each Scrum Master, and we have to revoke the rights again when they either are changing ART or Role. This will be a much easier task and you will get a great overview if you can assign rights to Member Groups, as we can just add or remove members the group and they will get needed rights from the group and not as an individual.