Vestigial organs are evolutionary souvenirs. They are not necessarily "useless organs," which is how some people interpret the term.
A structure is vestigial if it doesn't have the same purpose as it had in the past. Consider whale bones. They have bones in their fins that are shaped much like our fingers (well, all mammal's fingers, not just ours). They're not useless, they just have a different purpose now. What's important is that it shows that whales descended from land mammals with fully developed fingers.
Discussions of vestigial organs usually only concern vertebrates. All vertebrates have essentially the same body plan, so comparing and contrasting is very easy to do. Comparing a whale and a cheetah is more interesting than comparing a rabbit and a clam. There isn't much to say other than, "Yup, different. Anyone want to go to Chipotle?"
Vestigial organs are great for science because, essentially, every organism is dragging its evolutionary past around with it all the time. I tried doing that with travel souvenirs once, but I got some awkward stares for carrying around snow globes, spoons, and keychains all the time. But at least everyone could see instantly where I'd been, right? Right?