Most sequels exist purely on the basis of the first film making a truckload of money at the box office. You can't blame studios for wanting to cash in. Existing for no other reason than its predecessor earning over $600 million worldwide in 2008, 'Mamma Mia: Here We Go Again' is one of the most unnecessary films of the year. Nobody really needed to know the origins of Donna Sheridan and her young exploits which led to the failure of knowing the father of her daughter from three possible candidates....