The main benefits from the use of SSL:
Server with self-signed certificate: Protects from communication sniffing. Does not protect from Man-In-the-Middle attacks (MITM)
Server with CA-signed certificate: Protects from communication sniffing. Client can verify server identity. In most cases protects from MITM attacks.
Server and client with CA-signed certificate: Both sides can verify each other. This method provides mutual authentication.