One of the most recognizably "modern" features of modern architecture is the extensive use of windows, especially plate glass. This is really a pleasant thing, bringing the outside in and opening sightlines.
But why wasn't this always the case? It would seem like a no-brainer to have lots of light and enjoy the view of the outdoors. But there were cultural and technological changes that had to happen before we thought those things.
Glass was once very expensive and difficult to produce, and through the 1800s it was more feasible to make windows out of many small panes of glass, leading to the casement and sash windows we are familiar with in older period buildings, often double-hung in warm climates like Florida.
As industrial glass production improved, large panes were increasingly used, notably in the "Crystal Palace" building at the 1851 Great Exhibition, a precursor to modern architectural styles. At the same time, ideas about light and space were changing, slowly moving away from the Victorian idea of dark, curtained spaces towards what we would recognize as a "modern" ideal—light, bright and open.
At the turn of the 20th century, architects began experimenting with novel materials like pre-cast concrete, aluminum, and plate glass, producing structures that moved towards a full integration of interior and exterior spaces. Frank Lloyd Wright was especially known for this, using expanses of glass to complement the chosen sites of his buildings. Other architects followed his lead, and now the idea of a floor-to-ceiling window or clear wall of glass just seems another part of good design.