Specifying start-pos permits you to begin the search for search-string within, rather than at the beginning of, source-string. This is particularly useful if source-string may contain multiple instances of search-string and you wish to determine the starting position of other than the first instance.
Examples
=FIND("e", "where on earth") returns 3 ("e" is the third character in the string "where on earth").
=FIND("e", "where on earth", 8) returns 10 ("e" in earth is the first "e" found starting from character 8, the "n" in "on").
=FIND(REGEX("([A-Z0-9a-z._%+-]+)@([A-Za-z0-9.-]+\.[A-Za-z]{2,4})"), "Where does [email protected] start?") returns 12, the starting position of the first email in the source string.