It is to engage intensely the reader of the story by capturing his attention with a moment of active, momentous actions that will avoid useless exposition. The issue with providing context and characterization by telling the story in a sequential, linear narration is that if such details are superfluous or uninteresting or bland compared with the action sequence of a medias res narrative, the reader might quickly lose interest and close the book and never finish it.
By focusing initially on an intense action event at the beginning of the story the reader’s interest will immediately be engaged and alert and curious about the causes and origins of the event he is reading about. Once his initial interest is secured the author can then take his time to use analeptic storytelling (flashbacks) to provide the reader with all the details prior to the event which opened the storytelling narrative.