It's not always the publisher. It can benefit some middle parties which implement DRM. Or for example mobile carriers which use DRM to prevent users from switching. Or whatever other monopolistic lock-in scenario.
When publishers realize that monopoly falls to some other hands, they quickly become sober and find common sense. When monopoly remains theirs, they pretend that DRM is needed for other reasons.
How does this benefit the publisher? They already have a exclusive rights to whatever particular movie you're watching.