I think the point would be to do something like:

    a = 3
    b = 4
    c = 6
    "A is {a}, C is {c}, B is {b}".format(**locals())

Then you don't have to figure out specifying order or definitions like ".format(a=a, b=b, c=c)". I'd probably go the more explicit route in production code anyway, but for debugging / unstable development it seems useful.