because an organism needs even more cells to help it function better and change as it grows (like metamorphosis and yes, puberty :p) and if the same amount of cells stay then the organism's cells will be functioning and working harder.....so cells divide so many different ones can continue to work on different parts of the organism as it grows and changes. Plus it'd be weird if we only had like 1 thousand cells in our body from mothers' pregnancy the size of hi-polymer erasers (the big ones)..........
