1. A doula is a professional who has knowledge about pregnancy, labor, birth and medical procedures.
2. A doula will stay with you for the entire birth process, while nurses and other hospital staff come and go with shift changes.
3. Your doula works for you, not for the hospital.
4. A doula is trained to provide comfort measures that can ease labor and help prevent unnecessary interventions: physical things like massage, position changes, aromatherapy; emotional support and reassurance; and the simple, powerful fact of her presence.
5. You have a relationship and a rapport with your doula before your labor begins. She is someone you trust, someone who has learned about your history and your feelings around pregnancy and birth. She is committed to helping you have the best possible experience.
6. A doula can complement the vital role of your partner: giving him or her a break, acting as a second set of hands, reassuring and answering questions, and freeing him or her to support you as only a partner can.
7. Your doula is a care provider, not a family member, so she is more able to remain calm and think clearly if situations become stressful.
8. Prenatally, a doula can help you educate and prepare yourself for birth—through discussion, recommending books and videos, accompanying you to OB visits, etc.
9. A doula will follow up with you after birth, doing postpartum visits and staying in touch via phone and email to make sure you are settling in with your baby.
10. Doulas improve birth outcomes! (For example: Shorter labors, fewer Cesareans, even better success with breastfeeding, according to The Doula Book (Klaus, Kennell, Klaus)).

Here is a great summary of the evidence, including lots more reasons why doulas make births better.