I don't have an official answer for you and I don't know what it might be, but I can tell you what I’ve seen in Master Yang Jun’s school. I was in the first class he taught in Seattle and post standing was not introduced at that time. However, I attended a recent beginner’s class lecture by M YJ and he instructed them to begin post standing before they start studying the form the next week. He told them that in some traditional schools in China, a student is not taught anything until they can stand for the duration of one stick of incense burning (about one hour). And some train to two incense sticks (two hours).
I haven’t attended any other beginning class lectures so I don’t know if he’s introduced this before to other beginner’s classes; but he’s mentioned it before in the intermediate classes.
He demonstrated the opening posture, then standing with arms up in front (palms facing inward), then advocated Standing in the bow stance and empty stance. For the empty stance, the arms can be down, or they can be up in the end position of lift hands and step up, or the arms can be rounded in front (palms facing outward).
I’ve also heard other students ask him about it and, he says it’s a good addition to one’s practice. When I asked him for advice on staying calm in push hands, he recommended standing meditation so that I could more easily remember what it’s like to be still even when moving.
But then, I’ve also heard that tai chi chuan is sufficient gong even in and of itself, that it builds internal energy, gently clears chi blockages, and generally all the things nei gong can add. IMO, standing is a shortcut to internal understanding and development in the same way that scaling a sheer cliff face is a shortcut to a twisty mountain road. One eventually makes it to the top of the mountain, either through strenuous effort but shorter duration, or slowly and steadily on a longer path.