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From my understanding this would be beneficial if you've taken a stock engine and put a smaller, more restrictive air intake on it.
It would also be done to create more low end torque which shifts the power band down the RPM range.
What other reasons would someone reduce valve duration?
Restricting the airflow into an air-breathing engine cannot increase its power output. In the case of an internal combustion engine with valves in it, here is the critical fact you are missing:
It takes time for the cam system in an engine to push open and then let close the valves in each cylinder. As the engine spins faster and faster, it becomes progressively more difficult to get all the exhaust gas out of the cylinder, and a full fresh charge of air and fuel into the cylinder afterwards, in the time available.
To accommodate this fact, engines designed for high speeds have cams in them which hold the valves open longer ("long duration"). They begin opening the exhaust valve significantly before the piston hits bottom dead center and opening the intake valve before the exhaust valve is fully closed. This is called overlap and at low engine speeds, the overlap allows the burst of exhaust gas to blow backwards into the intake manifold and push the incoming charge out of the cylinder. This wastes energy and makes the engine run rough at idle and low speeds. This effect is so strong in high-revving motorcycle engines that they will not idle at speeds of less than 1000 RPM- but it allows them to produce peak power at 10,000 RPM or more.
The low-speed efficiency of an engine like this can be maximized by minimizing the valve overlap which also requires minimizing the valve duration. But at low speeds, there is still enough time to blow out the exhaust and suck in a fresh charge even with a short-duration, low-overlap cam.
For this reason, an engine that is cammed for low speeds (like a truck engine) produces significantly more torque at low speeds than does an engine which is cammed for high speeds (like a motorcycle engine). My Suzuki GS1000 motorcycle engine idles at 1200 RPM and does not "get up on the cam" until it exceeds 5000 RPM and produces its peak power (63HP) between 9,000 and 10,000 RPM.
A similar Suzuki engine as used in their small 4-wheel-drive jeep idles at 500 RPM, and produces peak power of 45HP at 5,000 RPM.
Could be a method of reducing emissions without modifying the engine, less fuel = less exhaust, albeit with less power as well.
An aside, how does reducing valve duration create more low end torque? Unless I'm misinterpreting your question.
