it makes the assignment visible to subprocesses.
jcomeau@intrepid:~/rentacoder/bin2txt$ foo=bar
jcomeau@intrepid:~/rentacoder/bin2txt$ bash -c 'echo $foo'
jcomeau@intrepid:~/rentacoder/bin2txt$ export foo
jcomeau@intrepid:~/rentacoder/bin2txt$ bash -c 'echo $foo'
bar
