Why does the nuclear envelope break down during mitosis?

The nuclear envelope breaks down during mitosis to allow for the distribution and segregation of chromosomes into the daughter cells. This process occurs to essentially facilitate the movement of chromosomes. During chromosome separation, the nuclear envelope disintegration allows the replicated chromosomes to be released from the nucleus into the cytoplasm. This assists them to segregate and align properly in the next stages of mitosis. The breakdown of the nuclear envelope also allows microtubules of the spindle apparatus to interact with the chromosomes. The disassembly of the nuclear envelope also ensures that nuclear content is evenly distributed among the daughter cells, and that they are genetically identical. On a side note, at the end of mitosis, a nuclear envelope is newly produced around each set of segregation and de-condensing chromatin.