Why was repeating the exposition important in many Classical sonata-form movements?

In Classical sonata form, the repetition of the exposition was a standard instruction because it was essential to the movement's structural and dramatic balance.1

Music is an art form fundamentally based on the controlled use of time, and the temporal proportions of a movement are critical to its overall design. By repeating the exposition, composers ensured that the listener was thoroughly familiar with the established themes and tonal landscape before the development section introduced conflict and tonal flux.12

When this repetition is omitted, the thematic balance of the movement is often upset. Furthermore, the dramatic impact of the development section—which relies on a sudden departure from the established regularity of the exposition—can be ruined if the listener has not fully internalized that initial stability. Consequently, the repeat serves to maintain the intended proportions of the work, ensuring that the subsequent "rigorous argument" of the development and the final synthesis of the recapitulation function as intended.13