Why were horns often used as harmonic filler in 18th-century orchestration?

In 18th-century orchestration, horns were frequently utilized as harmonic filler primarily because of the technical limitations of the instruments available at the time. Before the invention of valves in the early 19th century, horns were "natural" instruments, meaning they were restricted to the notes of the natural harmonic series.

Because they could not play a full chromatic scale, composers used them to reinforce the harmony in relatively slow note values or to provide body and texture to the orchestration. By using crooks—detachable pieces of tubing—to change the instrument's fundamental key, composers could integrate horns into various tonal contexts to fill out chords. While players eventually developed the technique of hand-stopping to alter pitch and access additional notes, the horn remained an essential color in the Classical orchestra, often doubling parts or providing sustained harmonic support to the melodic lines played by strings and other winds.1234