Movies are often seen as mere entertainment. Walt Disney, among others, made his fortune on that belief. But some of the best movies ever made don’t entertain so much as examine the social issues of their time. This week, Dan Webster, Mary Pat Treuthart and Nathan Weinbender discuss three movies that follow that latter formula: “Till,” the story of the lynching of the teenager Emmett Till, and its aftermath; “Descendant,” a documentary about the search for and salvaging of the last ship that carried enslaved peoples to America; and “Tár,” a narrative film starring Cate Blanchett as an embattled orchestra conductor.