Why might animals have vestigial structures?

The vestigial came about from the common descent and evolutionary theory. Vestigial structures are often homologous to structures that are functioning normally in other species. Therefore, vestigial structures can be considered evidence for evolution, the process by which beneficial heritable traits arise in populations over an extended period of time.

Over the period of millions of years, animals have went through many changes and the vestigial structures are forming. Even humans have about 6 vestigial structures ...