Ever notice how brightly-lit stairways are? This is obviously for safety reasons, but less obviously: each and every last corner needs to be illuminated to make sure you don’t step into (and trip within) a stray shadow.
Since railings are also essential elements for safe staircases, it makes sense to let them light each tread and riser while also providing visual guidance for landings and other changes in direction or shape as one descends.
Zoran Sunjic envisions this as an extruded, heavy-duty tube or LED light rope strung between supports and able to change color, potentially (for example) to illuminate what level you are on within a building (another helpful way-finding aspect). Still, as anything other than emergency lights, there may be one problem: too much illumination at eye level.