Light conditions during transition

[Grim Tuesday]

I've read that you should reduce a plant's light while it is transitioning to S/H. What is the logic behind this? I thought plants need light to produce mass (e.g. new roots) and will often invest more in their foliage if they are receiving insufficient light (but we we want roots, not leaves during the transition!) I am particularly thinking of the case of plants that like medium high light conditions, and received them at a nursery, so reducing light would be a change for the plant while keeping it at medium high light would be maintaining the status quo.