It should have an emotional impact. A great movie based on a true story makes the audience feel something. It could be inspiration, like in the case of 'Hidden Figures', where the story of the African - American female mathematicians in NASA makes you feel proud and inspired. Or it could be sadness, like in 'Hotel Rwanda' which tells a harrowing tale of the Rwandan genocide.
Authenticity is key. It should stay true to the main facts of the real - life story. For example, if it's a historical event, the movie should get the time period details right, like the costumes, the language used, and the social norms.