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. You can see the split beginning in the French repertoire with Debussy and Saint Saëns with pieces like La Mer and Saint Saëns' 3rd symphony. Both pieces by those composers where written for a section of F and C instruments. Mahler's 3rd asks specifically for a short, quiet passage in the third movemnt to be played on an Eb cornet. 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