Why in paragraph below, it is used "the participant" and "the reader" and "the writer"? Can't we write them as "A participant", "A reader", "A writer"?
Talking about “one of many” is also called “indefinite reference.” We use it when the noun’s exact identity is unknown to one of the participants: the reader, the writer, or both. Sometimes it’s not possible for the reader or the writer to identify the noun exactly; sometimes it’s not important.
Reference: http://writingcenter.unc.edu/tips-and-tools/articles/