Why are upper-level horn and trumpet players given a transposition sight-reading exercise? What is it like?

All horn and trumpet players seeking membership in the Vermont Youth Philharmonia or Vermont Youth Orchestra should prepare to play a simple sight reading excerpt in transposition. Players aiming to join or remain in Vermont Youth Sinfonia are not required to play this exercise (but may do so if they wish)
Transposition is an essential skill for orchestral horn and trumpet players since much of the pre-20th century orchestral repertoire is scored for horns and trumpets in varying keys
Excerpts to be sight read will be similar in character to classical-era horn and trumpet parts (e.g. Haydn, Mozart & Beethoven) – primarily triadic and stepwise patterns. Click to download example.
Horn players should be prepared to transpose to E-flat (down a whole step from F horn) and D (down a minor 3rd)
Trumpet players should be prepared to transpose to F (up a fifth from B-flat trumpet) and C (up a whole step)
While weakness in transposition skills won’t preclude a placement in VYO/VYP, students will be expected to improve those skills significantly to maintain participation in those ensembles