X25519 is the Elliptic Curve Diffie-Hellman (ECDH) function over Curve25519.

For Encryption/Key Agreement (e.g., ECIES, ECDH): You need a algorithm where two parties can derive a shared secret. For example, with ECDH, Alice uses her private key and Bob’s public key to compute a secret, and Bob does the same with his private key and Alice’s public key. The math makes this possible.