Asteroids, like comets, are leftovers from the formation of our solar system about 4.6 billion years ago. Despite their relatively small size, asteroids can be dangerous. Many have hit Earth in the past, and many more will crash into our planet in the future.
The 30 June 1908 Tunguska Event and the more recent 15 February 2013 Chelyabinski incident demonstrated the destructive power that these objects can unleash should they impact the Earth. This is one of the reasons why scientists study asteroids.