What benefit does the proficiency bonus the feat Weapon Master?

@SteinerDavion @Wizards_DnD in 5E what benefit does the proficiency bonus the feat Weapon Master provide to a Fighter proficient in all weapons none – it's really there for non-fighter types branching out

— Mike Mearls (@mikemearls) April 22, 2016

@SteinerDavion @Wizards_DnDwould it be overpowered if it let fighters paladins or Rangers double their proficiency bonus? yes

— Mike Mearls (@mikemearls) April 25, 2016