When you give the Viewer access to a group of people in the root of a structure, you could work with subfolders by 1) removing some people from access specific subfolders or 2) changing the type of access specific person would have. For example,
1) I have specific content that should be restricted but need to manage it in a single place. If I try to remove a person from a subfolder, it will remove this person from the whole structure.
2) I can create a root folder called "Teams" and give everyone the viewer access to it (since the content has to be shared) but then, I would like the leader of Marketing team having editor access to his/her folder since this person will be in charge to feed the content. I'm not able just to change his/her access from Viewer to Editor in that specific folder rather than I send him/her a new invitation. It would be simpler if I just change the type of access in that folder.