A Promise Creates an Obligation, Not a Forecast
A prediction and a promise can use the same future tense. They do different work.
It will rain tomorrow is a forecast. I will send the document tomorrow is a commitment. The first can be wrong without creating a broken word. The second changes what another person can reasonably organize around.
That is why promising is generative language. It does not cause the promised result by declaration. It creates an obligation for the speaker and expectations for the people relying on the promise.
Circumstances may later make performance impossible or inappropriate. The promise still matters because it calls for action, communication, renegotiation, or cleanup.
A promise should therefore not be confused with confidence. We can be uncertain and still commit. We can also be highly confident about a future without promising anything.
Prediction describes a future we expect. A promise places our word into the future and makes us accountable for how we meet it.