Longer rods provide for a longer dwell at piston TDC, keeping the combustion chamber small for a longer period as the expanding gases push against the piston. This extracts more power from a given air/fuel mixture thus increasing combustion efficiency. And so much like with good quench, you've once again raised the amount of compression that you can utilize for a given combination (and octane) before detonation becomes a concern. Also, much as does a stroker set-up, longer rods benefit the power band (read "torque curve") with the largest advantages in the low and mid-range RPMs. Even in a stock bore and stroke combination, longer rods can give you some of the effect of a stroked engine. And in fact on a shorter stroke, an even longer rod can be used within the given limits of practicality, this meaning pin location on the piston.
AND if that is not enough information for you; try this: