Why is internal fertilization necessary in reptiles?

Birds and reptiles evolved to lay shelled eggs that required fertilization to happen before the protective shell is formed. Internal Fertilization is one way to make this possible. Sperm, the male gamete, is a single cell with a flagellum that allows it to move or "swim" in fluids. Internal fertilization creates a contained environment in which the sperm can "swim" to the ovum, the female gamete.

In mammals, reptiles, birds, and some types of fish the gametes meet inside of the female's body. This is called internal fertilization. All land dwellers need to fertilize this way because sperm still prefer swimming to walking.