A summary level in a hierarchy has 2 purposes.
Summarizing the detail below. Add a summary level always summarizes the detail below that summary level. You can have as many summary levels you want by using the + icon on the last dimension item or right-click context menu.
Storing data inputs. Use Display to select Store data at all levels in hierarchy to show a data entry-ready item for the summary level. In the example First total, Second total and All List of names have both a data entry item and a summary level.
This feature is useful for adjustments, unallocated values and other summary level inputs.