The movie is primarily about the rise of the Casting Director as a prominent position in Hollywood, although it does a sideline in being a hagiography about Marion Dougherty.
In the 1930s and 1940s, Hollywood studios had stars on contract and casting - such as it was - was done by seeing who was available and who would be the biggest box office draw in the title role. Didn't matter that the character didn't fit the actor's style, put that round peg into that square hole ...
Marion Dougherty started arranging minor characters for TV in the 1950s. In the 1960s she was working in Hollywood as a Casting Director, fitting actors to roles she thought would suit them as the contract casting model fell apart. The movie interviews dozens of famous directors and actors to sing her praises, directors saying they couldn't have done it without her, actors saying they'd never have broken through without her. The latter seemed rather superfluous: they didn't interview people who she didn't cast, only the most successful of those she did cast. They interviewed only one director (Taylor Hackford) who downplayed the role of Casting Director, apparently as an incredibly weak attempt to show both sides of the argument - so instead Hackford came off seeming like a villain.
It was educational about the role of casting in movies, but the excessive love it showed for Dougherty (even if much of it was justified, which is hard to tell from within a movie so in love with her) was a little sickening.