A doula isn't a midwife and they do very different things. Midwives are medical professionals, so they're there to help you with the medical side, monitoring heart rate and BP, helping you made decisions about your care, and physically helping you to birth baby. Doulas are there to support you and your husband, but not with the medical side of things, so they do massage, help hold you up if you need support, bring you food and drinks, go out and take a walk with you if you need to walk around, suggest positions to try, take photos, and they can be an extra person to talk through any medical decisions you might be thinking of making (like your partner would be), but they can't give medical advice and they won't catch your baby. They're there for you, kind of like a sister or mother would be, but someone who goes to births regularly and knows what to expect and has some extra skills like massage, aromatherapy, etc. that you might want to use during your birth. Midwives are sort of there to assure your and baby's health and safety, doulas are there for your comfort and support, if that makes sense. Many people have doulas in midwife led births. I'd say actually probably more so that with OBs (as most people who'd want what a doula has to offer, wouldn't opt for ab OB led birth unless they had to).
I think if you want that sort of support, especially if you don't know your midwife well (you might not have your own personal midwife on call on the day), then a doula you know and have a relationship with can be great. I had a home birth with my first and I had two midwives and a doula. My experience with the doula wasn't great, just because she wasn't a great doula (but the only one I could find), but I wouldn't say that isn't an endorsement for doulas in general (my best friend is a doula actually, but she doesn't live near me, so couldn't be *my* doula). What mine did on the day was make suggestions for things I could do to encourage better positioning and progress in labour (my daughter was back to back and it slowed everything up at one point), she made suggestions when I was pushing for how to be more efficient at it, which did really help. After my daughter was born, she took my placenta (was having it encapsulated) and made sure it was put in the fridge, helped my husband to get our daughter dressed, made us cups of tea, put the birth pool away, and then cleaned up and put anything that needed to be washed in the washing machine. The midwives were doing all their monitoring, keeping notes on my progress, consulting with each other on medical matters, and doing all the post-birth checks, so she did all the rest. She also was the only one who took any photos as my husband and I were just too distracted and didn't even think about it!
They also usually check in with you for a couple appointments postnatally in the first two weeks just to see how you are doing, hold the baby if you need a shower or nap, light cleaning, support with feeding or other baby care, etc. But like you said you can just hire them postnatally too if you don't want anyone there for your birth and just want some help after.
I didn't have the most positive experience with mine (even though she did do all those things I mentioned about) just because she was sort of stress-inducing, very disorganised, and barely even showed up for my birth, but I think that was more of a personality thing and just we weren't a good match for each other, rather than anything to do with doulas in general. I think a doula is great to have if you don't mind an extra person in the room, especially if you are having a non-home birth (with mine, I could send her and the midwives to another room when I wanted to be alone, in a hospital or birth centre though, you'll have only one room, which means everyone who is there for your birth will basically sit in the room with you). I personally didn't like that many people around me. Midwives come and go, but in that setting, your partner and doula and any other family will pretty much be right next to you, and I wouldn't like that, but that's just me. If you're someone who prefers to be alone, then an extra person might be annoying more than anything. But I do think they are helpful if your partner isn't going to be super involved and hands on, like if he is someone who will support your weight during contractions, do massage, run to get you drinks and snacks, make sure the music you want is on, providing positive feedback, etc. then you may not need another person doing that. My husband was really hands on and I didn't really need another person around to do anything of that because he was already doing it. I am considering hiring a doula again this time, but only to support our daughter who will be there for this baby's birth. I want someone who can explain to her what's going on, take some photos, get snacks for her and I, give my husband a break when he needs to do things with our daughter, and then possibly be with her immediately after when we are distracted with the baby, make sure she eats her dinner, gets ready for bed, etc. Those are the sorts of things obviously a midwife wouldn't help with and you might have family step in and do, but only if you want family around for your birth (or they live close enough to provide that sort of support at a moment's notice, ours don't).
