If birth was a mountain, Doulas would be the Sherpas.
A doulas role is one of primarily education and then support. Her job is to help clarify options so her clients can make well informed decisions regarding what kind of care and birth they want and then doulas will do whatever possible to help them achieve their goals.
No matter how many books you read, how much advice you get, how many childbirth prep classes you take - you and your partner will probably not retain a lot of the information you will need to put into practical use when the time comes. You are both going to be emotionally invested and possibly exhausted and in need of support. Hiring a doula helps ensure that you can both remain present with each other and in the journey rather than feel like you need to be navigating the birth field. Brains work better in birth when they can be on autopilot, and a good doula will help you get there.
Birth in hospitals is quite new to us as a species. Up until it became the norm (which was really only about 100 years ago), almost every birth was attended by midwives or by other women. In other words, for all of time - until about 100-150 years ago, the wisdom of women, our bodies, and the wonderful midwives who helped along the way are how each and every one of us got here.
Midwives are childbirth specialists. When you have a specific issue, you would see a specialist. You wouldn't go to a foot doctor if you had issues with your bladder...and If you don't want birth through surgery, you generally wouldn't need to get your prenatal care from a surgeon.