Authentication is the ultimate requirement of encryption. Without it, you can't decrypt, making the entire mechanism useless. Amazing what lows you boys will stoop to.
Of course it depends on the system you're talking about. If you are doing public key encryption, then you do need a public key for people to encrypt to (but that's not used to get at data, only to encrypt it), and a private key to read it. The private key is the one that's important. If you enter that key in every time (but it's kind of long), the authentication is the fact that the key properly decrypts the data. You must be thinking of secret passphrases that you use to get into the program that holds the actual key for you (easier to remember a small passphrase than a 1024-bit key).
For authentication, you may be thinking of the feature of public key encryption that lets you sign and authenticate documents. This isn't hiding or protecting data from disclosure, merely verifying it.