The 34 day launch window for the OSIRIS-REx probe will open in about five weeks (early September 2016), and it's approach to the near earth asteroid 101955 Bennu (provisional designation 1999 RQ36) is planned to start about two years later around August 2018. Currently the plan is to stay with the asteroid until the next return to earth opportunity. The plan is to start back in March 2021, and return to earth in September 2023.
In the year 2135 Bennu may pass through a "gravitational keyhole" - a small point in position (and velocity) in this case between the Earth and the Moon. At that point, the future trajectory is very sensitive to the exact orbit through the gravitational fields of the Earth and Moon.
One distinguishing feature of this mission is the planned collection of 60 to 2000 grams of material and return it to earth for study. One reason for this might be that Bennu currently has some possibility of eventually striking earth in another roughly 150 years, and a wide range of information about the object will be valuable.
Photographs and various other measurements will collect information about the surface. According to Wikipedia, detailed study of the surface will be required to understand its interaction with sunlight. Details of the shape, reflectivity and rotational dynamics are necessary to predict the impact of solar photon pressure and the Yarkovsky effect, both of which need to be calculated using a realistic model of the asteroid rather than using that of a sphere with uniform surface properties.
Here is a nice presentation and paper on Bennu's orbit and study.
What are the types of measurements will be done? With which technologies will OSIRIS-REx scan the surface of Bennu? What exactly is hoped to be learned about the shape and properties of the surface?
above: subset of frames from the very nice 4MB GIF in this NASA webpage.
side note: While the Pioneer anomaly (discussed also in this question and its answer) was proposed to be related to anisotropy in the radiation of thermal energy due to (mostly) internal heat sources, the excellent analysis here illustrates the important of detailed models of heating, thermal transport, and radiation from a complex body in order to more precisely predict radiative (and also reflective) photon recoil.