Would it help to know her story, as a child she endured abuses including
domestic violence,
incest,
pedophilia, poverty,
racism, and
sexism. Granted, we are talking about her role as Celie Harris in the Color Purple. But she won the award for Best Actress in a Drama so you know this was so real for her that she could make it real to us as well.
Not only so but she grew up in a public housing near the cotton fields in Manhattan, so when she talks about racism she knows what she is talking about, it is as real to her as to any person watching a movie.
Also I would point out that she has stood up for others when they were maligned, she defended Michael Vick, defended Bill Cosby and defended Roman Polansky saying "it wasn't rape rape". This is true, what Polansky did was not "rape rape" I have no idea what "rape rape" is but he has never once been charged with "rape rape".