There are several effects in Photoshop that incorporate Global Light. These effects are those that require a simulated light source such as Drop Shadow. When Global Light is changed, any effect on any element which has this Global Light capability will be changed. This is very powerful as the effects on multiple elements can be updated by simply changing the Global Light setting in any effect. This is also where the potential danger lies as elements with Global Light capable effects that may not be visible or simply unnoticed will also change.