hook_help() argument I don't see the second parameter $arg being use in the function. 

function hook_help($path, $arg) {
  // ....
}


Why should I declare it? What is its use?
Take a look at the api. It states: 


  $arg An array that corresponds to the
  return value of the arg() function,
  for modules that want to provide help
  that is specific to certain values of
  wildcards in $path. For example, you
  could provide help for the path
  'user/1' by looking for the path
  'user/%' and $arg1 == '1'. This
  given array should always be used
  rather than directly invoking arg(),
  because your hook implementation may
  be called for other purposes besides
  building the current page's help. Note
  that depending on which module is
  invoking hook_help, $arg may contain
  only empty strings. Regardless,
  $arg[0] to $arg[11] will always be
  set.


Example:

function hook_help($path, $arg) {
    if($path == 'user/%'){
        if($arg[1] &lt; 10) {
            return 'one of the early users';
        }
        else {
            return 'foo';
        }
    }
}


Doing this without the $args would be ugly.