The Default feature allows you to mark Aggregations, Dashboards, and Diagrams that you work on frequently as the default entity of that type for your user. Once that is done, when you first go to the editor page for that type of entity, the defaulted entity will be automatically chosen from the list so you can view or edit it.

To set one of these entities as the default, choose that entity from the list on the appropriate page, then check the Default option next to the dropdown: