A mother told me this story. Her baby was a couple weeks old. The birth went well, but now a fever and redness on her breast worried her. She called her midwife who wasn't clear about what it was. Whatever she told her to do may not have been enough, or soon enough, because the fever grew. Her doula was able to stop by. The fever had grown to 104 and the red made a streak on her breast. She told her, if it were me, I would go into the hospital.
The mother had mastitis, an infection of the breast. Perhaps her milk had become blocked behind a tight bra strap. She didn't have time to tell me the complete story.
My point is, that there are times when a mother's care provider isn't completely available. Whether that is because it is between visits, after the usual visits, a shift change, or a busy day, there are times when the mother has to figure out her health needs herself. When a mom is sick she may not rally enough to initiate a course of action. She may not recognize how immediate her need is. Mastitus can flame up fast. She may not realize a small symptom can become a big problem.
The doula has a motherly way of keeping an eye on the mother. The doula is not a medical person. She doesn't diagnosis or treat problems. But very often she has experience in spotting something out of the ordinary. She can suggest the mother take the extra step to get the care she does need when something like this comes up.
The doula fills the gap.