My partner is supportive and involved, and has been preparing for the birth with me. Why would we want to have a doula with us?
Doulas (by many names...sister, grandmother, sagefemmes) have always been a part of birth in traditional societies. They offer a more objective and experienced flavor of support, and take the pressure off partners who may have their own strong emotional responses to labor, too. If the partner wants to give hands-on support to the mother, I can give quiet suggestions as to what may be helpful, and can ensure that snacks, drinks, and bathroom breaks are easy to obtain.
Having a doula present at your birth can actually increase the intimacy between the couple, as the partner is freed from the pressure to "coach" and can simply be loving and supportive in whatever way feels natural. Interestingly, studies have shown that new mothers have more positive memories of their partner's involvement in the birth when a doula was also present.