From the stories we’ve read so far, it appears that violence has made a name for itself in literature. It becomes an important aspect of literature in that it keeps the attention of the reader bated. Witnessing another human (especially a beautiful, young girl) being cut up into pieces in an unholy way hooks our attention on a vulgar level. It invokes a sense of dread to see what happens next and before we actually come to the said chopping of pieces, we sense it is coming. So, not only does this dread increase the weight of the story, it motivates us to read farther.
Without some act of violence (or threaten of imposing violence), there is no motive for rescue. For example, in the Robber’s Bridegroom we know already that there is some dark fate in store for the girl when her fiancé tells her his house is “in the dark forest” and he’ll mark the way with ashes. The sense that some grisly end awaits her at the house in the dark forest, heightens our curiosity and deepens our concern/interest in what will happen to the girl. (In this case, however, she confronts him and rescues herself.)
In Robinson Crusoe, the act of cannibalism (or his fear of it) adds a new element to the story. Cannibalism is what appears to be Robinson’s greatest fear on the island: for him, living in isolation for 24 years doesn’t seem to compare to the thought of being eaten alive and is what keeps him from venturing out to the main land. In this story, violence becomes the “added boost” to take the story to another level and provide the character (Robinson) a situation of even deeper danger. This makes the story to (some of) us more interesting and more worth reading.