Ed25519 is for signing or authentication. This curve is already available in OpenSSH (for authentication). Curve25519 is for encryption/decription.

Ed25519 has been available in GnuPG 2.1, but Curve25519 was not available.

Usually, we have a primary key for signing and certification (certification means signing someone's key in OpenPGP) and a subkey for encryption/decryption.

Now, Curve25519 is there in GnuPG 2.1.7. It means that we can create key of primary Ed25519 key and sub Curve25519 key.

Ed25519/Curve25519 is important because the curve is considered "safe" (if we compare somehow classic ECC of NIST P-256 or secp256k1).