Exact Difference between Predefined Roles "User" and "Viewer" in Narrative Reporting

Summary:

I found that most of the activities that user can do can also be performed by viewer based on the access provided. I'm really trying to understand which role the should hold.

Content (required):

As per the document below are the activities that User/Viewer can perform. Both are looking identical to me.

User: Views Narrative Reporting artifacts to which the user has access.

Viewer: Views reports and other artifacts to which the user has access. This is the minimum role required to sign in to and use an environment.