A better question would be why is internal fertilization used by land animals. As a living being we all started out in the sea. All reproduction must be conducted in an aquatic environment that can support the medium in which we live (again, water). read more
Within vertebrates, it is the amphibians and fish that use external fertilization. When it comes to invertebrates, most are benthic, sessile, or benthic sessile combined animals such as coral, sea anemones, and tube dwelling polychaetes. The benthic zone is the lowest level of the ocean where organisms called benthos reside. read more
One of several animal reproductive methods, external fertilization generally only occurs in aquatic creatures as water acts as a vehicle through which sperm can travel. The process involves the releasing of sperm and eggs -- or gametes -- near each other so fertilization can occur. read more