Well, I can certainly think of a few reasons. I'll cut to the chase and offer a handful.
Firstly, the single year of a masters does not compare to a single year in an undergraduate course: not only is the workload considerably greater, but postgrad research involves independently managing one's time without a supervisor pressing upon a student their obligations. That means a postgraduate student must possess determination, self-discipline, the ability to integrate short and long term time management, and a sincere passion for the object of their research. That really sets masters apart from ordinary undergraduates, and develops in them skills to carry for life, skills that do not also render them more employable, but actually improves their success at life - relationships, personal commitments, and one's pursuit of long term goals all draw from these qualities.
Secondly, having a masters *itself* immediately differentiates one from other graduates. It's very simplistic to propose this, but masters degrees are different and, well, impressive. That extra attention may make the difference between an application in the bin and an interview.
Thirdly, of course, masters degrees are simply more demanding, requiring intelligence, the ability to independently research and quickly assimilate complex ideas to an even greater degree than a bachelor's course requires. These are quite impressive traits.