One of the unifying features of architectural styles before the twentieth century is the presence of ornament. We speak of architectural elements as ornamental inasmuch as they are shaped by aesthetic considerations rather than structural or functional ones. Pilasters, column capitals, sculptural reliefs, finials, brickwork patterns, and window tracery are straightforward examples. Other elements like columns, cornices, brackets, and pinnacles often do have practical functions, but their form is so heavily determined by aesthetic considerations that it generally makes sense to count them as ornament too.
Ornament is amazingly pervasive across time and space. To the best of my knowledge, every premodern architectural culture normally applied ornament to high-status structures like temples, palaces, and public buildings. Although vernacular buildings like barns and cottages were sometimes unornamented, what is striking is how far down the prestige spectrum ornament reached: our ancestors ornamented bridges, power stations, factories, warehouses, sewage works, fortresses, and office blocks. From Chichen Itza to Bradford, from Kyiv to Lalibela, from Toronto to Tiruvannamalai, ornament was everywhere.
Since the Second World War, this has changed profoundly. For the first time in history, many high-status buildings have little or no ornament. Although a trained eye will recognize more ornamental features in modern architecture than laypeople do, as a broad generalization it is obviously true that we ornament major buildings far less than most architectural cultures did historically. This has been celebrated by some and lamented by others. But it is inarguable that it has greatly changed the face of all modern settlements. To the extent that we care about how our towns and cities look, it is of enormous importance.
The naive explanation for the decline of ornament is that the people commissioning and designing buildings stopped wanting it, influenced by modernist ideas in art and design. In the language of economists, this is a demand-side explanation: it has to do with how buyers and designers want buildings to be. The demand-side explanation comes in many variants and with many different emotional overlays. But some version of it is what most people, both pro-ornament and anti-ornament, naturally assume.