I know for us, my husband was fairly concerned that the doula would overshadow him. What exactly could the doula do for me that he couldn't provide? Why would we need one in the room? Birth is supposed to be shared by the parents...not some stranger in the room.
First and foremost, when you hire a doula to be a part of your birth team, you will likely find that they become a part of your family. You establish a trust and bond with them. They get to know you, your fears, and help to ease your transition into parenthood.
When a doula works with a dad, they reassure him that he is doing a phenomenal job at supporting his partner. We provide him tools and tips to provide hands on comfort to his partner while she is in labour, and we also provide him relief.
One of the best things that my husband praises our doula for is that she kept me entertained so he could sleep. He had had a long day at work, a tough workout at Crossfit that day, and a really spontaneous we are having a baby NOW moment, so it was really great to have our doula there so he could get some rest while we waited for my body to progress. When it was go time, I had the hands of two very supportive people holding my own, and cheering me on.
An unexpected thing that our doula did that I will never forget is that she took photographs of my husband holding our son for the first time. She was beaming with pride over his new-dad moments, and captured it for us to remember. It was amazing.
A doulas role is to provide emotional, mental and physical comfort to both parents during childbirth. This may mean grabbing a snack or a cup of coffee for Dad, or suggesting he go for a walk during a particularly long labour while the doula stays with mom. It may also mean playing interference with extended family members and giving the parents space. It could be as simple as verbally letting Dad know that he's doing a great job, and sometimes those simple reassurances are all that Dad needs.
Really...it means whatever the parents need for it to mean. A doula is there for support, never to overshadow.