Why we need to make new cells ?about biology
You have no doubt heard the little trivial factoid that the body regenerates itself every seven years. The simple answer to your question is that without creating new cells, the body would die. This is because individual cells (themselves) are dying constantly. The lifespan of a cell depends on what kind of cell it is - and of course on any extenuating stress factors (such as sickness, basic nutrician, drugs/smoking, etc). Because the cells in our body are dying at a consistent rate, our body is consistently replacing them with new cells.
Simple as it may sound, the making of new cells is also what makes us grow. Every single thing inside our body is made up of cells. If we never generated new cells, we would first of all remain the microscopic size of a combined sperm and egg. Not only is simple growth directly caused by the creation of new cells, but so is healing. Consider minor scrapes, cuts, bruises, broken bones, lost teeth, etc. Healing is the same thing as the body creating new cells.
This is why most scientists believe the cure for cancer (which are mutated cells) lies at the cellular level. Right now, we cannot cure the bad cancer cells. All modern medicine can do is wipe them out (zap them with chemo or radiation, or cut them out with surgery as in the case of tumors). While no one is ever considered "healed" of cancer - when all the cancer cells are gone and the body is generating new and healthy cells again, the person is considered in remission. This means there is still a potential for cancer to begin to grow again - but essentially it is all out of the body at the time of remission.