The key word is "string" as opposed to pattern. Fgrep, or grep -F doesn't do any regular expression matching. Consider searching for the string .*.* in a file:
grep -F ".*.*" filename
grep "\.\*\.\*" filename
See the difference? Using -F allows the search criteria to be free of any escape characters.
Also, if you are searching for strings, rather than patterns, I believe that using the -F option might result in quicker searches because there isn't any regular expression work involved with the scanning of each input record (I reserve the right to be wrong on that assumption).