I had a doula for both of mine.
You will essentially interview each other before a doula will take you on as a client, they want to make sure they mesh well with you first.
I also disagree with the midwife who posted higher up, sorry but I did not have good 1:1 care from a midwife with my first child, she buggered off most of the time and did not inform us about concerns with the CTG recording so it was a sudden shock when a Dr appeared saying they were concerned I might need an emcs due to the readings on the CTG that had started happening an hour ago and that we had no clue about. When I got moved to consultant led I thankfully got a new Senior midwife and he was fantastic and was present pretty much present the whole time for me. So, definitely no guarantee you will have a midwife present for you constantly, nor that they will communicate well with you.
In the above situation we were able to ask for a few minutes to discuss and during that time my doula was invaluable, she was able to give us more information from her knowledge and as a result we asked for further investigation instead of agreeing to an emca straight away (waters breaking and blood sample from baby) and in this way an unnecessary emcs was avoided.
My doula also was able to tell me about the Bishop score exam that can be done, when my consultant wanted to induce me at 38 weeks, and due to knowing about that and asking for one and being given a very low score, we decided against inducing that early (the lower the score the more likely a c section will be needed, he came naturally at 41+2 instead, I did go in for induction at 40+6 but was low priority so he came by himself before a bed in the labour ward was found for me).
Doulas will not advocate for you as such, they are not midwife nor medical professionals, so they cannot advise you on what course of action to take but they can help you understand your options and be a calming presence for both you and your partner. They are a fount of labour related knowledge. My first birth wasn't the best experience, but my husband was able to remain a bit calmer because he could see that my doula was not panicking, so a doula can be just as much support for your partner as they are for you.
They can help prepare you prior to labour, talk through any worries you may have, help you come up with birth preferences, discuss pain relief options and what you can do in the labour room to help you feel more relaxed and more in control (music, soft lighting, aromatherapy etc).
I forgot to take some frozen colostrum with me when we went in to have my first, so after he was born my doula kindly popped back to ours and fetched some for us.
My second doula was also amazing, with my second my husband couldn't be with me, partly due to limitations with covid and partly as he was the best option we had to stay home with our eldest. She gave me a lower back massage while I laboured, kept me company and chatted with me (I actually had her, the midwife and a student midwife with me for that one, but it was in a different country with amazing health care and they (the midwives) offered soft lights, music and aromatherapy oils to me themselves). She went and fetched me some food from what was left for patients after I had given birth and me and baby were sorted out. She also helped me book a taxi and met me at the hospital to help me and baby home after we were discharged.
Both doulas did after birth visits/debriefs with us as part of the packages we chose. We didn't feel we needed after birth doula care, both times we were more concerned about being prepared beforehand and having a doula present during the birth. I believe I have read somewhere that having a doula present during the birth reduces the likelihood of needing lots of interventions? Someone correct me if I am wrong.
If I had another (we are stopping at 2) I would absolutely be looking to hire another doula again.