To my experience the thread protector cuts down on wobble and helps to keep the can concentric to the bore. Knurled thread protectors are typically turned to size, then knurled, and knurling doesn't leave a precise outside diameter nor a concentric one. Hence the suggestion from certain suppressor manufacturers to use a smooth one.
I got mine from
GSL however they're OOS at this time.
There are other companies selling them, just be sure to get one that has an OD of 0.625″ for best fit and function.