Why does Sir Francis Bacon use the word "Of" at the start of every essay?
The English titles of Bacon's essays follow conventions dating back to classical antiquity and Greek and Latin titles. Before the Hellenistic period, ancient Greek literature was normally written on papyrus rolls. Information concerning authors and the title or subject matter of the book might be added to a small tag attached to the roll, but works did not begin with title pages the way modern works do. In fact, many of the "titles" we have for ancient works are simply labels added by librarians rather than official author-created titles in the modern sense.
For many non-fiction works, the labels attached to books on papyrus rolls simply indicated the name of the author and the subject matter. For example, "Longinus Peri [of, on, or about] Hypsous [the sublime]" just means "Longinus about the sublime" and "Aristotles Peri Poietikes" means "Aristotle on Poetics."
The titles for Bacon's essays reflect this tradition of simply naming works by the subject. The word "Of" is used in a manner meaning "on" or "about" to indicate that the essay contains Bacon's thoughts about a specific subject.