Some folks think they need to intricately fine scrub every nook and cranny after shooting their muzzleloader. A removeable breech plug permits the more [censored] among us to have better access. Sort of the Felix Unger vs Oscar Madison schools of muzzleloading.
I don't even pull my tough hole liners, pump em good with warm water and thoroughly dry and oil. Haven't ruined any guns in 40 years. I suspect that some guys are wearin their guns out with all the cleaning.