A floorplan ensures that cells belonging to a specific portion in the design netlist are placed at particular locations on the device. You can use manual floorplanning to close timing on a design when timing is not met using standard flows.
If the cells are not contained within a level of hierarchy, all objects must be included individually in the floorplan constraint. If synthesis changes the names of these objects, you must update the constraints. A good floorplan is contained at the hierarchy level, because this requires only a one line constraint.
Floorplanning is not always required. Floorplan only when necessary.
For more information on floorplanning, see this link in the Vivado Design Suite User Guide: Design Analysis and Closure Techniques (UG906).