I have my favorite quote as the answer for this question from the book by Klaus, Kennel & Klaus:
“Asking the fathers to be the main support, our society may have created a very difficult expectation for them to meet. This is like asking fathers to play in a professional football game after several lectures but without any training or practice games.
We (doulas) therefore want to enable the father to be present at his own comfort level and to remain emotionally connected to his partner and child.
For these reasons, every woman in labor needs not only the father or other chosen partner but also a nurturing, experienced person – a doula – who can calmly and skillfully help her cope with labor and be reassuring and constant presence for both her and the father.
Very often the couple worry that an outside support person will take over and control the labor experience. Though doula training is quite different, emphasizing quiet reassurance and enhancement of the natural abilities of the laboring woman.
A doula is constantly aware that the couple will carry the memory of this experience for the rest of their lives.”