The receiver uses this key ID message prefix to look up the secret key needed to decrypt the message. The receiver's software automatically looks up the necessary secret decryption key in the receiver's secret key ring. These two types of key rings are the principal method of storing and managing public and secret keys. Each user keeps his own pair of key rings. An individual public key is temporarily kept in a separate file long enough to send to your friend who will then add it to her key ring.
Some people buy it when they forget their password for their own files.
Law enforcement agencies buy it too, so they can read files they seize. I'm not as certain about the security of PGP as I once was about my brilliant encryption software from college. If I were, that would be a bad sign. But I'm pretty sure that PGP does not contain any glaring weaknesses. The crypto algorithms were developed by people at high levels of civilian cryptographic academia, and have been individually subject to extensive peer review.