Who is a support partner? That is different for everyone. You may want your partner, a relative, or a friend to attend your birth. Your support person is most likely very close to you. That closeness can make it very difficult for them to see you in such an intense experience. Many support partners have not attended a birth before, and having a doula present can be very helpful to them as well.
Doulas are not replacements for support partners. A doula can help the support person with techniques that help them feel like part of the birth rather than a spectator, and when needed, allow the support person the opportunity to rest during a long labor. A doula can also answer questions that the support partner might have during the process of pregnancy, birth and postpartum.