essentially, a green bell pepper is just a less ripe version of a red bell pepper. flavor differences do exist, but are somewhat minimal, a red pepper is somewhat sweeter, but with the same basic flavor. read more
In the places where I've lived green bell peppers are usually cheaper, usually half the price of the red. So that partly might explain the why. As for when, I think because red bell peppers are sweeter, certain recipes might not need that sweet flavor, it might need the zestier flavor of the green bell pepper. read more