Tonality is not merely a set of notes but an emotional field. 18th- and 19th-century composers felt the “character” of tonalities keenly: D major is festive and brilliant, B-flat minor is dark and tragic, E-flat major is heroic (it is no accident that Beethoven's Eroica Symphony is written in it).
Moving from one tonality to another (modulation) is one of the most powerful expressive devices. In the 20th century Arnold Schoenberg abandoned tonality, proposing dodecaphony — a system of twelve equal pitches. Yet tonality returned and lives on in pop, jazz, and neoclassical music.
