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 on a site, and then later be boosted to a higher role at the page level as part of a group (recommended method!) 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, you would assign them to a group with authoring or editing rights and grant access on a page. This way users can edit a specific section, while still only being able to view other public or semi-public pages.