You can use mental ray for:

"Usual raytracing"; although I don't know about the benefits. Some difference in sampling quality/settings apparantly. Maybe some difference in rendering speed? And handling of maps perhaps. I'm not sure.

"Final Gathering"; using lights or light emitting materials/maps to "fire off" the final gathering rendering "method". This is way cool, and makes realistic diffuse scenes.

"Global Illumination"; Using light(s) to emit a given number of photons to wash the scene and with some tweaking, resulting in a realistic diffuse (light) scene.

And then a combo of FG and GI, presumably to save rescources yet get the desirable result.

I hope this helped you out a bit.