As much as we may wish to believe otherwise, violence exists in the real world. Hollywood movies attempting to address that reality must show violence as it really is; to do otherwise is to minimize the effects and impact of violent behavior. For example, the movie "Saving Private Ryan" opens with an extremely graphic depiction of the invasion of Normandy. If the filmmakers had chosen not to show the violence as intensely as they did, they would in effect "sugarcoat" the actual circumstances of the invasion and distort the experiences of the soldiers who fought there.
In the context of fiction, violence can serve an important dramatic purpose. Its power and visceral nature can help drive home specific lessons or enhance key narrative moments. For example, Roman Polanski's adaptation of "Macbeth" presents the play's famous murders in a grisly and brutal fashion. That helps to emphasize Lady Macbeth's later guilt and madness at the deed, which is a vital aspect of her character and essential part of the story as a whole.
Some have argued that violence and violent images serve as a catharsis: helping us vet our darker emotions in a safe and harmless manner. Everyone experiences anger from time to time, or a desire to harm their fellow man (even if most of us know better than to act on it). To deny that side of ourselves is to empower it, but to release it in harmless ways--such as watching a horror movie or playing a violent video game--can defuse it. For example, many fans of the "Nightmare on Elm Street" series identified with the killer Freddy Krueger more than his victims, even though they never went out and slashed victims up with a razored glove.
In a society that values freedom of speech, violence and violent images cannot be censored, even if some find them distasteful. By extension, films and television shows that depict violence serve as an example that freedom of speech is alive and well, and that artists are welcome to express themselves however they wish.