It’s true that BArch holders do not need an MArch to become licensed architects (and in some states they don’t even need a BArch).
However, many BArch holders will choose to get an MArch on top of the BArch. The may wish to expand their studio exposure or add a degree from a prestigious school of architecture or focus on a specific architecture niche. At some firms the MArch is tacitly understood to be a requirement for partnership.
Over the years as architecture has become more technical and specialized, many schools have phased out their BArch in favor of BA/BS+MArch programs. As I said it’s a broad industry which encompasses several educational paths.