Why does a cell make copy of it DNA before mitosis occurs?

Mitosis is the process by which cells divide and make an exact copy of itself. If one cell has one organelle and divides into two cells, each cell would then have 1/2 of an organelle. This is the same for DNA. If the cell did not make a copy of its DNA before it divided (mitosis) each cell would only have 1/2 of its DNA and would not function. If the cell doubles its DNA it then has 2 sets of DNA and when it splits in half, the resulting cells then have 1 copy of DNA.