It is essential in much the same way that structure is essential to architecture.  We need structure to enable us to defy gravity, raise a roof, and make space.  We use daylight to apprehend that space as something inhabitable, a place that supports human life.

Electric light confirms the presence of structure and capacity. When you open the refrigerator door, the limits of the space are clear as are its contents, but nothing more is revealed.  Daylight brings with it a range of color to which eyes have evolved strong responses; and it changes, morning to night and through the seasons, enabling us to measure time; it then creates textures, through shadow, that also change, with time.  Buildings—spaces—change along with daylight, assuming various characters, changing moods, and suggesting a range of possible activities.  And when daylight disappears, there is night, dim light, and darkness, but usually with some spare illumination. In a world of daylight and nightlight, we know where we are, what time it is, and we are stimulated to find out by the necessary human need for visual variety and to satisfy our need to verify our orientation.

Electric light is only concerned with the one space in which it has been installed, but daylight connects us to all spaces, because that same light from the same source—the sun—is present everywhere on the planet.  All humans live and see in that same ocean of light. The energy that illuminates and warms Reykjavik, Iceland comes from the same sun that blasts Palm Springs, California. Architecture was invented—and continues to be invented—under that sun, and the way we build now reflects our ancient expectations as well as our evolved capacities to see and understand. The impulse to gather daylight, on surfaces and in spaces, to make shade and shadow are the same then, now, and everywhere.