If this is the Mr Base product you mean:

https://squadron.com/mr-hobby-mr-base-white-1000-bottle-40ml/

the label indicates that it "can make overcoat more vibiant (sic); can be used as a surfacer as well."

I'm not sure what the "overcoat" is in this context, and I certainly don't know how it can be made more "vibiant" (this reminds me of that line from "Beetlejuice", "This reads like stereo instructions").  Unless Mr Hobby means it's an undercoat as we would think of when we undercoat a color to make it brighter, like laying down white under yellow.  That it can be used as a surfacer goes to Jakko's comment about using Mr Surfacer as a primer as well as a surface treatment.  These products all overlap a little in their characteristics.  I haven't yet tried using Mr Surfacer as a primer, but more because I haven't really needed to and I haven't curious enough to try.  I can picture using Mr Base as a primer, too.

As an aside, I've always wondered whether in Japan, the brand is Hobby-san, with Base-san, Surfacer-san, and so forth.  I wouldn't be surprised to find that to be the case.