Why does DNA replication need to occur?
DNA is like the instruction manual for building and operating a cell.
DNA replication needs to occur 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 a full set of instructions!
Here is a video which uses an animated tutorial to explain the process of DNA replication.
Hope this helps!
Primarily for cell division
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.