Why Is the Exposition of a Sonata Often Repeated?

Classical sonata expositions often end with a repeat sign. If this is understood merely as “the audience may not have followed it, so play it again”, the structural work of the repeat is underestimated. An exposition does more than introduce themes. It commonly moves from the tonic to the dominant or relative major, establishing a tonal relationship that has not yet been resolved.

On first hearing, new melodies and turns of phrase naturally attract attention. On returning to the opening, listeners already know where the section is heading and can more readily hear how the transition leaves the tonic, how the second group gains a new position, and how the closing music temporarily confirms it. The repeat does not simply duplicate time: the same music now appears within an existing memory. When the development fragments, transposes or recombines the material, those changes have a clearer reference point. The recapitulation’s return of the material to the tonic can then sound like a genuine relocation.

The first movement of Mozart’s Piano Sonata in C major, K.545, is particularly clear in this respect. Its exposition is compact and its tonal movement easy to hear. After the repeat, the development’s departure and the recapitulation’s adjustment carry greater weight. A pianist need not impose dramatic differences on the second statement, but can clarify breathing, voicing and cadential direction. Whether to take a repeat still depends on the work, edition and scale of performance. Yet when the composer writes it explicitly, it is usually not a dispensable photocopy; it is part of how the piece organises musical memory.

Sources:
https://musictheory.pugetsound.edu/mt21c/SonataIntroduction.html
https://viva.pressbooks.pub/openmusictheory/chapter/sonata-form/
https://imslp.org/wiki/Piano_Sonata_No.16_in_C_major%2C_K.545_(Mozart%2C_Wolfgang_Amadeus)