Why is claude debussy important?

Because he created a new style of music which, much to his disliking, was termed impressionism. The disliking originated in the rejection of an early composition by a jury who described the music as being "vaguely impressionist", alluding to a contemporary painting school.
Debussy's new ideas, developed in the decades around 1900, helped French music, and modern music in general, to emancipate from the overpowering model of Wagnerian musical drama and orchestra treatment.