Q:

Why use os.path.join over string concatenation?

I'm not able to see the bigger picture here I think; but basically I have no idea why you would use os.path.join instead of just normal string concatenation?
I have mainly used VBScript so I don't understand the point of this function.

A:

Portable
Write filepath manipulations once and it works across many different platforms, for free. The delimiting character is abstracted away, making your job easier.
Smart
You no longer need to worry if that directory path had a trailing slash or not. os.path.join will add it if it needs to.
Clear
Using os.path.join makes it obvious to other people reading your code that you are working with filepaths. People can quickly scan through the code and discover it's a filepath intrinsically. If you decide to construct it yourself, you will likely detract the reader from finding actual problems with your code: "Hmm, some string concats, a substitution. Is this a filepath or what? Gah! Why didn't he use os.path.join?" :)

A:

Will work on Windows with '\' and Unix (including Mac OS X) with '/'.
for posixpath here's the straightforward code
In [22]: os.path.join??
Type:       function
String Form:<function join at 0x107c28ed8>
File:       /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py
Definition: os.path.join(a, *p)
Source:
def join(a, *p):
    """Join two or more pathname components, inserting '/' as needed.
    If any component is an absolute path, all previous path components
    will be discarded."""
    path = a
    for b in p:
        if b.startswith('/'):
            path = b
        elif path == '' or path.endswith('/'):
            path +=  b
        else:
            path += '/' + b
    return path

don't have windows but the same should be there with '\'

