I got this good question on Twitter from @MrJoshArless.
Asteroid 1998 QE2 is in our neighbourhood. It swung by yesterday afternoon and it's on its way back out, in its own orbit around the Sun. It'll be back, since we are all orbiting around the same Sun. This used to happen a lot more often in the early Solar System, as you can tell just by looking up at our Moon on any clear night, and looking at all the craters from impacts in the past. Planets like ours were formed by rocks just like 1998 QE2 coming together and forming bigger and bigger rocks, until these bigger rocks were massive enough to pull in more rocks, and eventually form planets.
As I answered @MrJoshArless on twitter, any two masses are attracted to each other, and it's even possible that this moon of this asteroid is just a piece of the asteroid itself, locked in an orbit around it because of the attraction of the masses, just like our Moon is to Earth. In fact, many people think that our Moon is made up of pieces of early Earth that have come together after a cataclysmic event.
NEOSsat is a recently-launched Canadian satellite whose task it is to search for asteroids and catalogue them, especially those that have an orbit around the Sun smaller than that of Earth's. It is hard to see these from the surface of the Earth because you need to look towards the Sun during the day to see them (most of the asteroids you see at night will have orbits around the Sun larger than that of Earth's). Knowing how many are out there, classifying them, and understanding their orbits will go a long way to predicting how many more 1998 QE2's are out there.
These asteroids are remnants of the early Solar System, they are hugely interesting from a scientific perspective because they can tell us about the building blocks of our own planet and of the other planets. They have been "frozen" in time, they are just as they were billions of years ago when our planet was in its infancy; a very different place than it is today. Some of these asteroids can even have organic molecules on them, including the building blocks of life. Everything that is on Earth today came from asteroids and comets some time in the past. Canada is actually participating in the OSIRIS-REx robotic mission to an asteroid to bring back a piece of an asteroid to Earth, to study a "fresh" sample. You can also learn a lot by studying meteorites that fall to Earth, but they are sometimes changed by the atmospheric entry process, by being contaminated by the pile of dog poop it lands on, or just by handling by humans.
Some asteroids can even be rich in what we consider precious materials here on Earth. That's why some people have an eye on mining them for their materials. What if a rare material here all of a sudden became abundantly available? A resource can only be considered a resource if one can sell it for more than it costs to get it. Can you actually make money mining an asteroid? Some people think so and are moving ahead with efforts such as Planetary Resources to do exactly that.
Super cool stuff. Thanks for the question, @MrJoshArless, and keep them coming.