3rd argument to os.path.walk(): Why?

[James T. Dennis]

  In the _Python_Essential_Reference_, by David Beazley
 (New Riders Publishing, 2000) I notice that os.path.walk()
 takes three arguments which he lists as:

	path, visitfunc, arg

 It notes that the function referenced in visitfunc is called
 with (arg, dirname, namelist) for every directory that is traversed
 by the os.path.walk.

 I don't understand what the "args" is for.

 I can see how to use the other arguments.  Here's a couple of 
 trivial examples:

	import os
	def print_dirnames(a, dir, flist):
		print dir

	def list_directories(a, d, l):
		for each_file in d:
			print "%s/%s" % (d, each_file)

	os.path.walk("/", print_dirnames, '') 
	os.path.walk("/", list_directories, 'x') 

 ... these both work as I expected.  Of course I'm simply
 ignoring the 'args' argument in my functions.  I guess I 
 could write my functions to behave differently based on 
 the contents of the args.  (I can even imagine that as
 useful, though I don't need it for anything right now).

 At first I though that this "args" element was required
 to be a string.  However, I tried calling the os.path.walk() 
 with numeric and anonymous (literal) lists and saw no 
 differences in the output as a result of those.   I guess
 I could call it with a lambda function or a reference to a
 function if I wanted to.  I don't know Python well enough to
 show an example, but I could see where that might be useful.

 However, my question boils down to:

	Why is this third argument required in os.path.walk()?