Why does external and internal fertilization exist in nature?

For external fertilization, the female and the male form a huge number of germ cells in their bodies. This happens in order to breed as many offspring as possible, since in the external environment the death is very high. With external fertilization, the egg and sperm merge in the external environment, and not in the female’s body.
With internal fertilization, male reproductive cells enter the female body through the copulatory organs. And the offspring develops in the mother’s body. This fertilization is necessary in cases where the female and male body is not able to form a large number of germ cells. Internal fertilization in such cases makes it more likely that fertilization will be successful.