Why does someone always type at the bottom of the article (PS:***)?

PS is the abbreviation of postscript, which means "comment"

Postscript, modern English-Chinese comprehensive dictionary

Noun (short for noun)

(Sign at the end of the letter) In addition, and; P.S., P.S. , PS or p.s.

Appendix (to a book, etc); postscript

Conclusion (after the news broadcast)

Postscript of Modern English-Chinese Dictionary

Noun (short for noun)

(Sign at the end of the letter) There is also a postscript, in addition.

Appendix (to a book, etc)