Why does DNA replication occurs?
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.
I'd also recommend visiting the following sites which give a nice overview of the process of replication and why it needs to happen.
Hope this helps!