The knob is used to relief the stress you might experience when you want to turn the know of the shower thermostat (temperature control).
You may suffer from arthritis - or perhaps the thermostat is just worn and difficult to turn - or maybe it'll help you children to better grasp the knob when it's wet?
In my case I removed the original (right) cover and pressed this knob onto.
I haven't tested it, but I may very well fit on the left side too?
If it dosen't fit yours then try to scale it up/down.
I experienced that the knob after some time started to come loose. So, I decided to make the inside hole a bit deeper (ShowerKnob_v.2.stl) - this secures it better as I slides further onto the thermostat.
3DE Premium PLA
The knob in the photo was printed in 0.25 layer height (to speed up the process).
I would suggest the use of support - depending on how you turn it you'll probably get different results on the surface.