I just don’t get it. Mufasa’s only role in the story is to die as motivation for Simba. This is like making a movie about Batman’s parents or Uncle Ben. Nobody cares.
I think they wanted to explain Scar's motivation for what he did in the original, despite the fact that it was already explained, he was waiting for his brother to die so that he could become King, but then his brother had a kid and he was bumped to second in line to the throne!