rainbow-delimiters-mode and syntax-begin-function

[fatiparty]

I am going through 

(define-minor-mode rainbow-delimiters-mode

There is for instance 

;; Obsolete equivalent of `syntax-begin-function'.
(when (boundp 'font-lock-beginning-of-syntax-function)
      (set (make-local-variable
            'font-lock-beginning-of-syntax-function) nil)) )

What is this about?  And how is one modifying `syntax-begin-function' ?