One thing to realize is that a Doula is very schooled in the knowledge of Childbirth - labor, all its phases and stages, its nuances. Also all the terms and measurements the Doctors will be using to decide how ready you are for labor, such as your Bishop's Score, which includes the station of the baby in the pelvis, the situation of the cervix, it's effacement, & dilation and what all that means in concert. She knows what labor positions to use when, to help keep labor moving forward, to help keep the baby in an optimal position, to alleviate some of the pain you're feeling in labor as well as to help keep the stress off of the baby. It's nothing against the husband. He is there for intimately personal emotional support. That can never be replaced! ;)
A Doula also knows many tips and tricks to use to help keep you focused away from the pain and stay informed and educated fully on what is actually going on so that you can truly make informed decisions when you need to at a critically emotional time. Doulas can also help you form a list of Birth Goals, or what is usually called a Birth Plan. She can help you find a way to put together a polite and compact list that seems non-invasive to the Doctor/Midwife & Hospital Nursing Staff but still serves your needs as well. She can also help you become more of an informed advocate for yourself, by teaching you how to talk to your Doctor/Midwife about things that you're concerned about or may disagree with in an assertive but diplomatic way.
Doula's Support both of you emotionally, educationally, physically, and mentally! They do not give medical advice or do anything clinical, like exams or blood pressure. And they support you in whatever decisions you make. It is your birth. They are there to educate and inform on all the risks and benefits, which unfortunately you sometimes do not get the full picture from Doctors-for many reasons.... That is not to say that they are a bad Doctor! No! But they do not train in "Normal, unmedicalized births." Things like not rushing to induce unless there is eminent danger, because it can often lead to a C-section. And many other things she can help educate you on so that you can fully weigh the pros and cons yourself before things get emotional.
And the more prepared you are, the less anxiety and fear will be present. Fear and Anxiety together are the top causes of long labors. The more relaxed and calm you can stay, the more likely your labor will move more smoothly. True no one knows exactly what will happen during labor. But the deck will be stacked in your favor with a Doula, leaving your husband to focus solely on you and the baby, knowing you have your own labor support person to stay with you from early labor in your home-where you should labor as long as you can, throughout the entire birthing process and even a little after to make sure you get started off on breastfeeding and bonding well! No shift changes! She's there for all 3 of you and serves your interests, not the Doctor's or the Hospital's.