You can extend the expiration date of (your own) keys as you wish by setting a new one, even after it actually expired. For this reason, setting an expiry date does not increase security against stolen keys. Regarding decryption of already encrypted files, this is irrelevant anyway as "the math" does not care for the expired key, nor do the implementations of OpenPGP.
The only advantage is "lost" keys (you haven't got a revocation certificate, uploaded the key to the key server network and lost the private key) expire after some time and don't stay on the key servers forever.
For a more in-depth discussion, have a look at "Does GPG key expiration add to security?".