What is the use of hard pen in Ling Fei Jing for daily writing?

It didn't work.

Including Biography of Ji An, Tao Te Ching, Ten Notes of Caotang, Biography of Laozi, Jiuchenggong, etc., will have no influence on daily Chinese characters. The reason is that our daily writing speed is very fast, we use Lian Bi characters, and the initial practice of calligraphy is regular script.

Just writing regular script instead of running script will become a situation where you write carefully and look good, and when you are in a hurry, you will let yourself go. Running script is not a quick writing of regular script, nor does it have to be adhered to. It's no use insisting on writing a million words. Once the foundation is laid, writing a running script, such as a few thousand words, will greatly change the daily words.

Lingfei passed by

Ling Fei Jing is one of the famous lower case letters in the Tang Dynasty. Chu and Xue were famous calligraphers and collectors at that time. He personally collected hundreds of original works by famous artists. Ling Fei Jing is vigorous in brushwork and exquisite in font, and is often used as a model for future generations to learn lower case letters.

Refer to the above? Baidu Encyclopedia-Ling Fei Classic