It will help much less with a single strong source of light. This will typically help more in scenes with lots of lights with a finite area of influence, like a cityscape for example. Roughly, what is taken into account to discard a shadow ray is its maximum possible incoming irradiance, which is linearly related to the light's intensity and decreases with distance. The specific number will vary depending on your exposure and light intensity values, but the default value is reasonable for the default exposure/light intensity values. This makes sense because below a certain threshold there will be no perceptible difference between shadowed and unshadowed areas. The point of low_light_threshold is to save casting shadow rays when Arnold knows that the error from not casting that ray is below a certain amount. Raising this value can speed up rendering by allowing Arnold to ignore tracing a shadow ray for light samples whose light contribution is below a certain value.
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