First, it helps to have a little history of the actresses involvement in the show. Majel Barrett stared in the pilot of the first Star Trek as the character Number One. This was huge, it was a female officer, second in command of a starship. In the episode she is seen giving unquestioned commands, leading a rescue mission, and just generally keeping her cool under pressure. She had potential, I think, to be a really interesting leader, but the show never took off.
When the show was revamped for a second pilot, Majel Barret played Nurse Chapel, the character in contention. Nurse Chapel is seen as the head nurse, assisting Bones in surgery and patient care. She is a capable woman of science and advances, through the movies, to a ship doctor and later works at Star Fleet Medical.