A Viewer is someone who needs immediate access to company data to generate reports used with any visualization tool such as Tableau, Crystal Reports, Etc.
This user typically doesn't know the data model, nor do they care. They just want to be able to pick data elements from a list and publish them to their visualization/reporting tool.
They "Shop" for data looking for what they need. Once found they can add the data fields to their own list of favorites, group them into categories of their liking and even rename the fields they use on a daily basis.
Without DSF, this user would traditionally need to contact the IT department to build an SQL Query for them to access the data they need.
A Viewer can source their own fields for reports based on what a higher licensed user has setup for them.