My understanding as explained to me from Jose Rubio, is to protect the chamber on a new pipe, especially for a new pipe smoker, that might smoke the pipe to hot, before it forms a carbon layer in the chamber to protect it, as it could develop hair line cracks in the chamber if smoked very hot.
There’s also the other school of thought, that these coatings might also help with a cooler smoke from the beginning, and might also help aid in the break-in process.
I personally don’t care for coatings, I have no need for them, and I find on nice, higher end quality pipes that are uncoated, the briar imparts a nice wood/nut flavor, and to me personally smoke as a cleaner tasting pipe. But that’s not to say on high end pipes, those pipe makers make bad coatings. People like Tom Eltang, Mike Sebastian and Former coat pipes, it all depends on who’s doing the coating.
So in the hands of pros on fine pipes, it’s just a subjective like/dislike is all. You don’t hear people complaining their Eltang tasted crappy. LOL ?