It looks as if they are intended to provide extra force for stretching the knee. Which of course also means that you will have to use additional force for bending the knee.
Anyway, if we accept the premise that this would be sensible, the next problem appears:
The rotation centre of the device will have to be in the same position as the rotation centre of the knee. Otherwise, the device will have to slide along the leg when you bend the knee. From the photos, it is very clear that the rotation centres are not aligned.
So the device will have to slide, and it doesn’t really seem fit for sliding. So you will probably get a lot of chafing.