So these woukd be good for e.g. EK Predator 240 AIO? But why wouldn't they make good case fan replacements if intending to run at same speed as previous case fans?
Yes, but I wouldn't bother replacing the EK Predator fans. They are a side trade in terms of performance, so only change if you want different aesthetics.
As far as new case fans, you can slow these down if intended to run as case fans but that would be a big waste of money tbh. I wouldn't be surprised to see a slower speed version of these sooner than later, and perhaps also new AF style fans also.
Worth getting to replace SP120's on a radiator?
Looking at knocking some noise down.
Yes, definitely. You will notice similar performance at lower noise if you slow these down, or get a lot better performance at similar noise levels (Assuming you have the SP120 HPE or the SP120L fans).