You start creating constructs when you want to assemble the basic objects of your building project into base files.
Typically, you begin a project by creating constructs. Adding optional elements often comes later in the process. For example, perhaps you know how many offices you need on a floor before you know the exact furniture layout of the offices. Therefore, you would first create the construct for the office walls and doors, First Floor—Office Space. Later, you could create elements for desk/chair combinations or generic printer/copier layouts and reference them into the construct with the office walls and doors.
Plan view of construct with highlighted furniture elements