I have been building and designing a hobby-tier game engine and I am currently working on supporting any amount of lights of any type. I checked out Unity and it supports multiple directional lights, what would that be useful for? Doesn't one directional light (without shadow map) already light the entire scene?
There are some Use Cases:
day & night cycle with separate sun and & mooon (don't forget to turn off shadowing once the sun/moon is below horizon ;) )
Or just multiple suns ;)
A different use case could be to have a ground diffuse reflection. When the sun hits the ground, it reflects some light and causes objects to be indirectly illuminated by the ground. (don't forget to turn off shadowing of the fake lighting ;) )