Kosher salt, or popcorn salt as I recall. Has more to do with the lack of iodine
I've used regular iodized salt in the past, and it worked fine, too. Not sure where the insistence on using kosher salt came from.
The difference between table salt, kosher salt and popcorn salt is how course the grains or flakes of salt are. In order from finest to coarsest it goes: popcorn salt (very fine so it can coat the kernels), table salt (we all know what it looks like) and kosher salt (bigger chunks, originally used to leach blood from meat). These aren't the only sizes/forms of salt but they are common.
Knowing this, it makes sense that people's go-to for leaching tars and nasty stuff out of the chamber of a pipe is the same one which was originally used for leaching blood from meat. Kosher salt may not be required, but it is the standard due to form factor and tradition. Also it's easier to clean out of a pipe bowl.