Further, there are probably too many people on the newsgroup for you to encrypt the message with all their public keys. Let's say that Moe is posting an announcement to the alt.fan.stooges newsgroup, and he wants to ensure that everyone can authenticate him as the sender of the message. Now all alt.fan.stooges readers know that Moe is the author of this post, so long as they have Moe's PGP public key on their public keyrings. PGP will authenticate that Moe's signature is valid. But let's assume Larry and Curly don't have such a tool and have to do it the old-fashioned way at the command line.
Public key is required to check signature. .