I've had doulas at both of my births- one was, TBH, mostly useless and the other I sincerely believe the universe placed at my birth bc I can't fathom how I would have gotten thru my birth w/o her....
My first birth was a hospital birth and, tho I was young and dirt poor, I knew I wanted a natural birth and that I wanted a doula. I found a doula in the process of starting up. I believe I was her 2nd doula birth. I paid her 50 dollars. She wasn't worth that much.

We met once pre-birth, to go over my birth plan and for her to give me a crash course in labor-relief. In retrospect, the fact I had a doula was a crutch for me to feel "safe" that I would have the birth I dreamed of.
I don't fault her now (I was upset for a long time), but my ferocious pitocin-induced-posterior-baby-prolonged labor-high forceps birth overwhelmed her and she basically stood in the corner and looked scared and meek. I'm not exaggerating. I'm a labor screamer and she freaked. My MW sent her home at some point, bc she was obviously not helping.
To this day, I believe my birth might have gone differently if I'd had a more experienced doula by my side; one who could have offered suggestions when I was lost in the pain of back labor, or positions that might have helped my OP baby turn, or more effective pushing techniques.... anyway... it was a good lesson in 'you get what you pay for'.
My second birth was a homebirth, and I *almost* didn't hire a doula bc I had a fantastic MW and was way more personally prepared for labor. However, my doula taught my birth class and I just fell in love w/ her presence. I hired her at 34ish weeks?
Like I said, I believe fate put her at my birth. I went into labor mid-morning and my DH called my doula. She came by just to say hi and help set up... but it turned out, I was in the throes of labor. She held me and centered me and chanted w/ me and yelled w/ me... she helped me get in the tub and then, immediately out when it felt like crap...and, well, long story short... I had a precipitous birth and I would have *NEVER* called my MW, had it not been for my doula's insistence I was in transition, just a mere hour after labor started (I was). MW arrived 4 minutes before DD was fully born. I would have been okay w/ a UC, except that I had a MAJOR hemorrhage w/ my placenta. Not only that, but bc my MW's assistant didn't make it in time, my doula assisted my MW at stopping my bleed, giving compression and meds and oxygen.
After I was stable, my doula made a gorgeous placenta print and tucked me and DD into bed

I'd say, if you do hire a doula, certainly pick one who you mesh well with and one with experience. With what I know now, I'd interview a million doulas to find one I clicked with, bc that's who you'll be comfortable w/ when you're in the throes of laborland.
I WISH I had had a pp doula. I'd say that would be incredible if you can afford it.