People in Tai Chi say: "we stand perpendicular to the ground because that is what you do in Yang Style", "we don't stand perpendicular to the ground because it is Wu Style". On the humor side, I met a Judo teacher who criticized the Wu Style "tilt" and her rationale was "Because we have hips". I have not heard functional justification for standing perpendicular to the ground in postures.
By functional we mean a coherent and comprehensive rationale directly related to why one should stand perpendicular to the ground. Frequently we hear: "Yang Style always stands this way", "my teacher said so", "it is for centering", "it aligns the acupuncture points with the ground", etc. and it leaves us wondering.
This article referred to Cook Ding's Kitchen: September 2009. https://cookdingskitchen.blogspot.com/2009/09/ As Wustyle.com in Hong Kong reports: "In 1916 Grand Master Wu Chien Chuan, along with other famous Wushu experts of the time Yang Shao Hou, Yang Cheng Fu, Hsu Sheng Chi Tzu Hsiu, Sun Lu T'ang, Liu En Shou, Liu Tsai Chen, Chang Chung Yuan, Tong Lian Chi, Chiang Teng Tsui, Hsing Shih Ju and others established the Beijing Institute of Physical Education."
These most famous teachers from "Yang Style", "Wu Style", etc., taught under the same roof. If I had to venture an opinion, I would say the teachers themselves were not going around saying, "I'm teaching Yang Style and because of that, the correct way to stand is perpendicular to the ground". My other opinion would be that it was not teachers but it was students who came up with the names, like: "I'm studying something from Yang", "I'm studying something from Wu". I would also venture another opinion and say, those gentlemen did not develop their arts in a vacuum...they also trained with each other, compared arts, pushed hands with one another, etc. That itself has profound implications. Reference to this can be found at Cook Ding's Kitchen: September 2009 (cookdingskitchen.blogspot.com)
Finally, I quote Master Hwa in answering a student's question:
Q. Why do you emphasize "classical" Tai Chi; and not "Wu" Tai Chi?
Tai Chi Health Benefits. https://www.classicaltaichi.com/tai-chi-health-benefits.html According to my teacher Grand Master Young Wabu, he heard his teacher, the legendary master Wu Chian Chuan said that Wu did not change the Yang Tai Chi learned from the Yang’s. In fact, Wu Chian Chuan emphatically said, "It can not be changed".
The fact is that the knowledge cycle of learning the classical Tai Chi to understand its eventual consequences is very long. To learn the Tai Chi form takes several years to be proficient. To test the effectiveness of what has been learned in actual martial art application takes another few years. To confirm what the health implications are, especially during older age, requires a human generation. Here we have the classical Tai Chi with its numerous components meticulously optimized to satisfy both the requirements of martial art application and health benefits. It must be a multi-generation effort. When Wu Chian Chuan taught my teacher, Tai Chi was already in such an advanced state. One can fully appreciate what he said, "It can not be changed".