Since community members cannot be boosted to pro member roles at the page level, the Viewer role allows a pro member to have limited editing rights in a site, and then later be boosted to a higher role at the page level as needed. The most common use case for this role would be if you had a user who you would like to create and edit content in a specific section of a site.
In this case, you would create them as a pro member with the Viewer role in the control panel. Then, on a specific page you would boost their role to an Author or Editor as needed.
This ensures they can edit that specific section, while still only being able to view pages that are public or semi-public.