Who is on your support team? How often will that person be available to you? Doctors and OBs stop in to periodically check on Mom, but they are not a constant in the labor room. Nurses, tend to have more than one woman assigned to them at a time and therefore have to be in and out caring for everybody. Midwives are likely to be more present, but often times do not join you until you are in active labor. Partners are a great source of comfort during labor, and they are an invaluable part of the birth team but they don't always know the ins and outs of labor. Doulas are there as a constant source of comfort and support from when you first need them until you're ready to settle in as a new family of 3 (or 4, or 5, or...), and they know labor. They are there to support both of you physically, mentally, and emotionally.
The medical field is constantly updating and changing to best align with what the latest research shows. Doulas stay up to date on the latest findings involving labor and birth. When making decisions about your medical care you should always be informed! Doulas can provide you with the evidence based research so that you can make those informed decisions. They also teach you how to ask the right kinds of questions to get more information if you're unsure about a suggested procedure.
This is YOUR birth experience, not your doula's nor anybody else's. A doula's job is to empower families to make informed decisions, and to advocate for themselves to ensure that those decisions are respected.