I am moving from RSA to ECC for my application.
Looking at these posts 1 2 3, they all suggest that Alice generates a temporary (ephemeral) ECC keypair eKP to send a message to Bob. The sessionkey sK is then generated at Alice' side as (privateKey eKP * publicKeyBob). The publicKey of eKP is transmitted along with the sK encrypted message. At Bob's side, he can calculate the same sessionkey sK as (publicKey eKP * privateKeyBob) and decrypt the message.
What I do not understand is why using a temporary (ephemeral) keypair eKP is better then just generating a session key sK directly from (privateKeyAlice*publicKeyBob).
Is it because we would re-use the same session key every time? Is it because we would have to pre-agree on seeds or initialization vectors for directly generated session keys, adding more interactions? Any other reasons?
Any insights greatly appreciated.