I'm hoping to get one again, I had one after my first surgery in 2019 in the hospital and then 2 weeks after at physio twice a week for 20 minutes. I found it quite relaxing, never too forceful and for me it helped enormously. Unfortunately they are too expensive to purchase.
My surgeon ordered a cpm machine but another one in his practice doesn't. Not having had a chance to compare, I can only speak to what I had. I think it was marginally helpful for the first 2 weeks. The key for me was to use my own judgment, listen to the knee and not go crazy with it. I never used it as many hours as they suggested. I used it mostly as a "warm up." I did notice that at the end of a session I was bending more than at the beginning. But that didn't translate into daily gains.
I think there is a tendency to involuntarily tighten your muscles against discomfort doing things like heel slides, so maybe the theory behind the passive motion is to keep your muscles more relaxed while taking your knee through its range.