Because when a parent cell divides in to daughter cells then it gives equal number of chromosomes to all the cells. Not only the number of chromosomes should be same but also the DNA should be the exact copy of the parent DNA. The reason is that DNA carries message which should be same in all the daughter cells. For example, if you are growing and your skin expands and parent cells of skin divide to form more daughter cells, then it is necessary that all cells have same DNA. Therefore, DNA replication is very important otherwise there can be abnormality.
And a single base pair substitution can cause the wrong ammino acid to be produced which will cause the shape of a protein to be off (eg. Sickle Cell Anemia)
It needs to be equally divided