It's like not-perfect-but-still-good forward secrecy: Your old certificate may be leaked or decrypted, but it will only endanger past communications; the communications after this one will still be secure.

Another side-effect is that you don't need to manage revocation stuff as diligently, because certificates automatically expire shortly. The window during which a certificate is valid is extremely short and recent, which means there is less chance that a problem happens. when that probability increases (as a result of being older), certificates become automatically invalid.