Have you ever noticed that most stairways tend to have a light switch at the bottom and another one at the top? This is done for more than convenience.
In the United States, the building code stipulates that stairways must have these twin switches, at those locations, if there are more than six stair treads. So, if it's just a quick three steps down to the next level, two switches aren't needed. Most stairways that go all the way from one level to the next, though, will encompass far more than six treads, so proper lighting and two switches are required.
It's also important for property owners to maintain these lights at all times. For instance, if the weather protection failed on a light at an apartment building, then the entire fixture could fail as a result. The exterior stairway could end up dark. This creates additional slip and fall hazards in addition to those that already exist. These could include poorly maintained treads or slick treads from rain, for instance . The lack of lighting could cause a tenant in the building to slip and get hurt.
In another situation, a light going out results in a tenant noticing the issue. Not wanting to take the dark stairway, the tenant decides to go to a separate and normally unused set of stairs. These weren't intended to be accessed and were not maintained, and the person was hurt trying to go up them.
These cases show why proper lighting is so critical, and that's why the code stipulates that lights with dual switches have to be installed. People must be able to benefit from them when ascending or descending. When lighting is inadequate and people are injured, they need to know what legal rights they have.
Source: Inspectapedia, "Stair Lighting: Guide to Lighting Requirements & Codes for Stair, Landings & Building Exits or Egress Routes," accessed Jan. 22, 2018