Shuts it off more, increases the depletion region. About the only time anyone intentionally does this on an NPN is with an avalanche transistor, an exotic trigger application used for its fast risetimes.
A more common use for reverse-bias is with the varactor diode. This diode is used for its capacitance, which is controlled by the thickness of the depletion region. You don't want it to conduct, so it is reverse-biased, and the more it is reverse-biased, the lower the capacitance becomes.