nat in pf

[Jason Hunt <leth_(_at_)_primus_(_dot_)_ca>]

This question came to my mind earlier today, and is probably really
stupid, but here goes;  Why does the 'nat' action in pf need to know the
interface to be applied to?  Since 'nat' is told which IP to rewrite the
packets' source address with, does the interface even matter?  Is the
interface given so that pf knows right away which interface to send the
packet out on, without having to look up what interface that IP is
assigned to?  Can you use pf to rewrite packets from an address that is
not even assigned to the machine?  I've tried following the code, but
still couldn't figure it out.