Originally Posted by Manny Laureano

The C trumpet was built when the big F trumpet and the Bb/A trumpet and cornets were the main horns that were used. The small Eb trumpet/cornet was also in use. The C trumpet was desgned to aid in the upper register demands of "new " repertoire like Mahler symphonies.

It became the favorite instrument of the French tradition while the Germans and Austrians and Finns like Sibelius continued to write for the F and Bb. Most of the French literature was written for the C trumpet. When French players came to America (Georges Mager, principally) they brought the sound of the C trumpet with them. This influenced players like Vacchiano and Herseth. Vacchiano was probably the first important player of his generation to use the C and D almost exclusively.

He used it to make technical licks easier and for the compactness and focus of the sound. So, to answer your question, yes, it's about transposition, focus, and sound. Many orchestral tuba players use a C tuba for the same reasons but I've never seen a C trombone.

ML