Question

why do animals have certain useless organs in their body

Answer 1

Snakes don't have legs, right?  but look closely! Pythons and boa constrictors have tiny hind leg bones buried in muscles toward their tail ends. Such features, either useless or poorly suited to performing specific tasks, are described as vestigial. They are also intriguing evidence of the evolutionary histories of species.Vestigial legs are a clue that snakes descended from lizards. Over 100 million years ago, some lizards happened to be born with smaller legs, which, in certain environments, helped them move about unencumbered. As generation after generation survived and reproduced, this new form flourished. Over time, all members of the group were born with shorter legs, and eventually with no legs at all. Almost. The ancestor of boas and pythons retained very small vestigial legs, a trait passed on to its descendants, including the reticulated pythons.

Answer 2

Vestigal organs are those organs which once had a function in animal body.They are no longer important so the body puts less energy into developing them,but they still inherit the characteristics in their DNA.