This limits the time during which the corresponding private key will be available to an attacker. The latest versions of PGP allow linked signature and encryption key pairs, but gives them the same validity period. Linked keys with different expiry dates are handled correctly, however. This allows us to implement software which can interoperate with PGP users. Signature keys that are long-lived and certified by other users allow a web of trust to build up.
Encryption keys are short-lived and certified by the user's signature key.
This solves a problem with previous versions of PGP, where creating a new key pair meant you lost all the certifications of your previous public key. An X.509 system would require separate certificates for encryption and signature keys, and the ability to retrieve certificates based on their keyUsage extension. This is not supported by most X.509 storage formats. Taking short-term keys to their logical conclusion, a different key could be used to protect every message. After a key was retrieved by another user, it would be deleted.