Why do tigers have claws?

Because of self defense
Yes, but that's not all. They need their claws for hunting, fighting, as a way to mark mark territory, to protect themselves and possibly their cubs. They need claws for the same reason that every other organism has any other sort of defense; self- and species-preservation. A tiger's claws is just one way of protecting itself and its species. Claws aren't great against a bullet or a blade, but they're great against everything else in nature. We're basically all built for ensuring the survival of the species ('all' meaning everything on the planet that has a hint of life in it). Everything has it's own way of defending itself; we have our brains (how messed up is that) and teeth and powerful arms and legs; raptors (birds, not dinosaurs) have incredible eyesight, speed, talons, and flight; plants have all sorts of different ways of protecting itself. Think of every inconvenience you've encountered from a plant; that was most likely one of it's defenses. Trees have bark to protect its insides; citrus fruits have peels to protect the seeds and fruit.