What is the primary purpose of crossbar lights?

The primary purpose of crossbar lights is indeed to help pilots determine if their wings are level on short final. These lights are typically installed as part of visual approach slope indicators and serve an important role during the landing phase of an aircraft's approach. When aligned properly, the crossbar lights can indicate to the pilot whether the aircraft is flying straight and level relative to the desired landing approach angle, thereby assisting in maintaining control during this critical phase of flight.

The other options, while they describe functions related to various lighting systems at an airport, do not accurately capture the specific role of crossbar lights in aiding pilots with visual reference during landing approaches.