Well, it's not like the unlit parts must be completely black.
Having some dynamic lighting does not mean that you must make it part of your game's mechanics. It's perfectly fine to do that kind of thing just for polish.
It's usually quite important if you want to create AI that behaves convincingly in a 2D world. The previous game I built using a 2D lighting system used it explicitly for this (other than making the environments look cool, of course) - the same geometry and light source data that lit the environments and produced shadows also determined whether enemies could see objects at given locations and whether you had hidden yourself by vanishing into the shadows or behind a given surface.