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Why are shades, blinds, and shutters used on windows?

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The Great Court at the British Museum in London features a large central rotunda surrounded by wide steps and a glass and steel lattice roof, with visitors walking throughout the space. — Architecture The Great Court, designed by Foster and Partners, 2000; in the British Museum, London.

In architecture, shades, blinds, and shutters serve as functional devices used to manage the interior environment when window placement alone does not provide sufficient control. Their primary purposes include:1

Light and Glare Control: These devices allow occupants to regulate the intensity, quality, and direction of natural light entering a space.12
Privacy: They provide a necessary barrier between the interior and exterior, ensuring visual privacy for occupants.13
Temperature Regulation: By covering windows, these treatments can help maintain a more constant interior temperature, acting as a buffer against external heat or cold.4

While windows are the principal means of controlling natural light, they often require these additional treatments because they are less effective than opaque walls at insulating against heat transfer. In contemporary design, if a building's fenestration is not carefully planned for light and privacy, shades, blinds, or shutters are considered essential functional elements to overcome these deficiencies.231