It's about options.
If a junior design position or paid internship can be landed at a reputable design firm, then this is of course the preferred route to take at that point in time. But that is a lot easier said than done, especially for someone who is coming fresh from a tangent design field and the primary career goal at the moment is not to work within education or research.
If there is no such opening, the masters can be a great thing to do instead for a number of reasons.
First, it surrounds you with peers who will inspire you, motivate you and teach you. Something that you probably will miss if you work on your portfolio on your own.
Secondly, it gives you the opportunity to really focus on an area of design that interests you and gives you the time, resources and freedom to explore. A luxury that one rarely has when working at a design firm.
From my own experience, I can't emphasize the fact enough that it is paramount to have a clear idea of the direction that you want to take. I don't think a Masters should be a continuation of a Bachelors, i.e. a skill drill. It is much more a time and opportunity to take learned skills and use them to dig deep into a subject matter of interest, creating new knowledge.
So, I actually might advice that, should there not be a great job on the table already, to consider another Bachelors. If another Bachelors is too much of a financial burden takes too long, Umea in Sweden for example offers a 1 year crash course in ID which quite a few friends of mine have attended and they later used to get into top notch ID master programs.
If you have to pick up basic skills during the masters education, it will distract you from the actual purpose of the masters and leave no time and energy for developing a deeper perspective into design as you would in a Bachelors.
I did a second Bachelor in ID after I had earned one in a another field, but dropped out before senior year and right away enrolled in an ID masters program. Personally, I could not have done it in another way and am very happy with the course I took, as wonky as it may seem.