Question

why is appendix in human considered as vestigial?

Answer 1

because it is of no use in the body

Answer 2

Vestigial in that sense that the organ seemingly has no functions or useless now but they might have  functions previously which are lost someway due to evolution. The human appendix may be considered  as a vestigial organ as it has been proved that the removal of the organ after infancy does not create any  harm.