Basically, every time a cell undergoes mitosis (one kind of cell division), various enzymes work to split each DNA strand in half, and then replace the missing half on the separated strands with corresponding nucleotides, leaving you with two identical strands. When the entirety of a cell's genome is copied (along with all the organelles), the cell can split into two daughter cells.
Imagine slicing yourself down the middle and splitting yourself in half, and then using each half of you as a template to recreate the other half.
That's the biology behind it, but the bottom line is that DNA is replicated in order to reproduce itself.
Dna replication needs to occurs because existing cells divide to produce new cells. Each cell needs a full instruction manual to operate properly. so the dna needs to be copied before cell division so that each new cell receives full set of instructions. - Google
In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction.
