Groups allow us to analyze our data on an aggregated level that is not available in the raw dataset we are using. By grouping, we have created a new group and field. When we insert it into a view and a measure (in this case, Employment), the measure is aggregated on the level of group. This means that, if we use the Average function, Tableau returns a single average value calculated across all the rows that belong to the same group member (in our case, Decade).