The technique of collage was pivotal to the evolution of cubism. To understand this it helps to start at the beginning. The term cubism was originally coined by the art critic Louis Vauxcelles in 1908 when he reviewed an exhibition of Georges Braque's paintings using terms like cubes, and Bizarreries cubiques' intended as ridicule. But the name stuck and the rest, as they say, is history, or at least art history. These early cubist paintings began to break down the picture plane into geometric shapes or "cubes".
The formal elements or composition of analytic cubism began to take precedence over its subject matter. The work became increasingly geometric, abstract and analytical so was called analytic cubism. Braque and Picasso both returned to subject matter without returning to representational naturalism by bringing divergent elements together into motifs thus synthesizing them, into the work. Hence, synthetic cubism. They began doing this by actually bringing bits of wallpaper, newsprint, oilcloth and even wood paneling into these paintings making collage a pivotal technique in the development of cubism.
Sources: Art History Vol II, Marilyn Stokstad, 1995; Oxford Companion to Art, ed Harold Osborn, 1970; Collage, Clement Greenberg, 1958.