Question

Why do cells multiply?

Answer 1

Multiplication of cells is absolutely necessary for 
 increase in growth, with reference to the increase in length of tissues
 regeneration of repaired and damaged cells.

Answer 2

it is a process of  cell cycle. they (cells) need to  replace themselves with fresh copies. large part of the cell cycle is called Mitosis. Sometimes there is a mutation and malfunction and cells multiple out of control - this is what we call cancer.