Why is the existence of vestigial structures significant to the study of evolution?

it's kinda really complex you have animals like snakes evolution says they used to have legs how can this be proven? This is proven by vestigial structures as some snakes like certain pythons still have legs however, they are underdeveloped and are used for nothing. Whales have pelvis bones this may not seems special but it is because every chordate that has ever walked on land has one thing in common and that's a pelvis this along with other data gathered really concrete the idea of evolution into reality