What is the use of private static member functions?

I was looking at the request parser from the boost::asio example and I was wondering why the private member functions like is_char() are static? :

class request_parser
{
  ...
  private:
    static bool is_char(int c);
  ...
};

It is used in the function consume which is not a static function:

boost::tribool request_parser::consume(request& req, char input)
{
  switch (state_)
  {
    case method_start:
    if (!is_char(input) || is_ctl(input) || is_tspecial(input))
    {
      return false;
    }
    ...

Only member functions can call is_char() and no static member function is calling is_char(). So is there a reason why these functions are static?