The nuclear membrane disappears during a specific phase of cell division, particularly during mitosis (more specifically, during prophase). The main purpose of the nuclear membrane breaking down is:
So that the chromosomes can move around the cell.
During prophase, the chromatin condenses into distinct chromosomes, and the dissolution of the nuclear membrane allows the spindle fibers to access the chromosomes for proper alignment and separation during metaphase and anaphase. This process is crucial for ensuring that each daughter cell receives the correct number of chromosomes.