Quite often the clarinet players influenced composers directly - this wasn't always just a professional relationship only, but often a friendship. It helped both: The musician needed new and popular pieces to play, and the composer benefited from finding out what a player could do with the instrument. Most composers then and today play the piano and maybe one other instrument, the more they know about the characteristics of the instruments, the better they can set music for them. You find a lot of such relationships between composer and player: Stamitz and Joseph Beer, Mozart and Stadler, Spohr and Hermstedt, Weber and Baermann, Brahms and Mühlfeld, as well as Benny Goodman who worked together with modern composers a lot.