It's given in my book that borax is a useful primary standard for titrating against acids. I have heard that primary standard is a pure reagent and does not gain any weight by absorbing moisture. (Hence any error during weight measurement is prevented).
I got this from a website that came first on searching "primary standard"
Primary standard is a reagent that is extremely pure, stable, has no waters of hydration, and has a high molecular weight .
But borax (represented as $\ce{Na2B4O7.10H2O}$) has water of hydration.