Ding, ding, ding! Horseshoe crab blood has an autoimmune reaction to any contaminates within about an hour, so it is a very quick and reliable way to check medical drugs for contaminates. They used to use live rabbits (I think?) and this process took like 2-3 days, so this is why horseshoe crab blood is so valuable!
By the way the horseshoe crab species is about [350 million] years old. Many, many other species depend on them to survive.
Video: Crash: A Tale of Two Species | Watch Nature Online | PBS Video