Cells divide for three reasosn: growth, reproduction and repair.
Onion root tips and whitefish embryos presents cells that's always undergoing mitosis.
Onion root tips are always growing to get more water and nutrients from the soil, therefore mitosis is always occuring to duplicate the cells and maintain a growing rate.
Whitefish embryos have cells that are undergoing mitosis rapidly, as it is a phase of intense growing and formation of the embryos to become fish.