The chromosome must be duplicated before mitosis.
Mitosis is where a parent cell divides and produces 2 genetically indentical daughters cells. This process first requires the parent call to duplicate all of its genetic material. This ensures that the daughters cells are genetically identical to the parent cell and also the other daughter cell.
This is important because mitosis is used in many cases in living things such as growth, repair, and more. This ensures us for example not suddenly changed to another organism or another characteristic.