Why such a mission? So we can learn about the origins of the solar system from a 4.5 billion year old rock.
Edward Beshore from the University of Arizona, who serves as the Deputy Principal Investigator for OSIRIS-REx, stated:
We are going to Bennu because we want to know what it has witnessed over the course of its evolution...Bennu's experiences will tell us more about where our solar system came from and how it evolved. Like the detectives in a crime show episode, we'll examine bits of evidence from Bennu to understand more completely the story of the solar system, which is ultimately the story of our origin.
The OSIRIS-REx mission will also be a step towards NASA’s Asteroid Redirect Mission (ARM). The mission of ARM will be to capture and redirect a near-Earth asteroid to a stable orbit around the moon, at which point astronauts can investigate it. This is seen as a first step before sending humans to Mars.
This video will explain more about the mission. You can also learn more about the NASA mission here.
Update: OSIRIS-REx was successfully launched on Thursday and has started its trip to Bennu.