A common concern for partners is that they will be replaced in the labor room; that suddenly in this life altering moment they are not adequate enough to support their wife.
A doula can never replace a partner. Ever. There is a bond between the two, a connection between these two souls that no one can replicate. The doula is part of the birth team, actively working to make the experience well remembered for both of you. Partners need to be freed up to emotionally support the mom, not worry about recalling the birthing techniques learned in class. The more relaxed and less pressured the partner feels to "remember everything" the more comfortable the mom will feel, because the atmosphere is peaceful.
There are different ways partners and doulas can work together:
For a partner who wants an active role in coaching, the doula can help with hints, reminders, and advice so the partner can provide the support for the laboring mother. "Is she having back pain?" your partner doesn't have to be well versed in techniques; the doula can offer suggestions while your partner puts them into practice.
Sometimes labors last a long, long time. Partners and doulas can tag team, so mother is never left alone. Your partner may need a quick nap, or the doula can run out and get the support team food. It never hurts to have too many players on your field.
Your partner may just want to experience the birth. They will be there to emotionally support the laboring mother, but wants to go along for the ride with her. The doula can be mom's sidekick during early labor, if that is what your partner chooses.
All these options work, and it is important to discuss ahead of time how partners and doulas can best work together, creating the most supportive team for mom.