First of all, I think from the point of view of using a pen, it is to make the pen tip not easy to scatter during writing, so that the hand can better control the brush, and the calligraphy and painting will not be ugly because of the deformation of the brush.
On the other hand, from the perspective of written characters, the central stroke makes the basic stroke outline of characters smooth and form lines, but if it is not the central stroke, there will inevitably be many "sawtooth" in the stroke outline, which is a big taboo in calligraphy and another meaning of using the central stroke.
Incidentally, China advocated Confucianism in ancient times, which emphasized the golden mean. The central stroke is not only the embodiment of the golden mean in calligraphy, but also the symbol of the golden mean with the characteristics of strokes. The so-called "impartiality as the center, impartiality as mediocrity". Speaking of which, let's talk about Yan Ti:
Yan Ti pays attention to the pen on the opposite side and the pen in the middle, which is the most exquisite font in Tang Jie. During the Anshi Rebellion, Yan Zhenqing was promoted as a leader to fight against the rebels, loyal to the monarch and patriotic all his life. Therefore, no matter how he uses a pen to be a man, it is in line with the Confucian golden mean. In addition, the font of Yan style is dignified whether it is enlarged or reduced, so the plaques of later generations, such as "XX Hall", are often dominated by Yan style.
But in regular script, you need to adjust the front when you start and close the pen. If the front adjustment is not good, the pen will become a black "dead knot", which is very ugly. And the so-called "want to go up first" means to start writing before the center, which is the trouble. This one is not as fast as the European-style exposed front and slightly sideways pen.
European style was born in Weibei, and Weibei paid the least attention to the comparison of the center, but paid the most attention to the side attack, which was influenced by the minority cultures in the north and the western regions and the ancient Indian culture at that time. At that time, these countries or regions often used "plain pen" to write characters, especially Sanskrit and some western languages. This pen is like an oil painting pen now, like a brush; So when people wrote Chinese characters at that time, they all put the writing brush down obliquely and then brushed it there in the crosswind. This way is very rough, and it is fast and beautiful to write. Now Wei Bei's remains are mostly epitaphs.
In the Qin Dynasty, Li Si's calligraphy style was the most standard. This calligraphy style must be written by the center, because only in this way can all strokes of each word be as thick as before, and each stroke is always as thick. The strokes of this word seem to be made of iron wire, so it is also called "iron line printing" Li in the Tang Dynasty was good at this book.
Orthodoxy cursive pen is changeable and eclectic, and it is also commonly used in the side, so-called "dragon and snake fight for the pen end"; But I have also met people who only use the center or mainly write cursive scripts. Smooth and full, well written and beautiful.
Throughout the history of the book, the middle stroke is a very old and traditional stroke, and I think it has its own advantages and disadvantages with the side stroke.
The above is my understanding, I hope I can help you.