I guess the best way I could explain it is that a Master of Science is a way to get your feet wet, if you want do do research. Probably you will not become a full-fledged researcher, but you will take the first steps. It is certainly a head start if you want to do a PhD. You probably will be better able to grasp what a PhD will entail, and have better chances during the PhD application.
I certainly learned a lot of things during my Master of Science courses and research projects. But, for me, the most important lesson I learned is that I should not trust the things I studied so far. Several theories I learned while studying for my bachelor's degree and my MBA are, if not completely wrong, at least incomplete, misleading, and very open to doubt. Some theories have been very twisted or simplified for undergrad teaching, for example.
If you want to have a glimpse of what I'm talking about, just compare what is taught about the "Maslow's pyramid" and the actual paper Abraham Maslow wrote. Although there is a resemblance, the paper is lot more detailed, interesting, and richer than the way we learn about his theory in school.
That skepticism is very important if you will follow a career in academia. That may change your life. If you used to trust all that knowledge you earned, now you might change to a permanent state of disbelief.