Why must DNA replicate exactly?
DNA replicate exactly so that after cell division each daughter cell will receive complete set of genetic information and by this traits are preserved.
In DNA replication, DNA makes copies of itself after cell division. Thus, if DNA replication does not occur then newly formed cells would eventually die.
DNA replication is semiconservative which means that only one strand of a DNA molecule is newly formed and other strand is original DNA which is made from the parent cell.
In mitosis cell divides itself into two identical daughter cells. For identical cells, same parts or organelles should have full instruction sequence to work properly. DNA should be copied before cell division so that each new cell receives a complete set of instructions.
If there is any mutation in DNA strand then cell division stops until it is repaired. DNA replication has to be completed before this phase as once mitosis starts then cell is already prepared for division.