Can you tell me why I need -o -?

[Oscar Levi]

I'm calling gpg from another program, redirecting fd's so I can pass
the passphrase, the message, and the signature between the programs
without writing temporary files.

I find that 

  gpg -sb --passphrase-fd #

will accept the message on standard input, but it won't write to
standard output.  I replace # with the fd I use to write the
passphrase.

  gpg -sb -o - --passphrase-fd #

does the right thing.

  gpg -sb --passphrase-fd 0 < inputfile > outputfile

lets me simulate the circumstances of my application, and this works,
too.  I first type the passphrase at the console and then I enter a
message.

I missing something obvious?