Why the blue whale have teeth?

The statement in the question is not correct. Blue whales, the largest animals on Earth, do not have teeth. Instead, they have baleen plates, which are made of keratin, the same protein that makes up human hair and nails. These baleen plates hang from the roof of the whale's mouth and act as a filter, allowing the whale to strain small prey such as krill, copepods, and small fish from the water.