It's no good producing a show if no-one knows it's on. Marketing includes all forms of media that encourage people to see the show - posters, newspaper ads, flyers, facebook, website, enewsletter, banners, flashmobs etc.
A good marketing campaign will have a theme look or image/message that is carried out through all media forms to strengthen the message. Having a good eye for design and creativite thinking is a huge advantage. Marketing may also be required to produce the show's programme.