An access group in AMS V5 is used to easy manage the access your personel has in the system and can also be used to get a list of selected users in documents.
You can i.e in controlled documents have a field of Process Owners/ Document Owners, here you can make an access group and add the persons you want and not give them any more access than being shown in the dropdown in the document.
Or you can use Access groups to all access in the system i.e workflows for modules for approval. With the use of Access Groups you dont have to set access on each module on each person, just add or remove from groups for fast access changes in the system.
In i.e NCR & Incidents you most likely have 3 steps, Registration, processing and verification, with access group setup in a good way you can easy set persons who will be able to process NCR´s and maybe other persons that can verify them.