This is a previously planned action.

This is a spontaneous or unplanned decision.

Making a promise

We use 'going to' for predictions based on previous evidence but we also use 'will' for general truths or predictions.

This is a previously planned action.

This is a spontaneous decision.

Will for a promise.

When we make a prediction or guess about the future, we can use either "will" or "going to" equally.

We only use "will" (or "won't) if we are refusing to do something or expressing (un)willingness.

This isn't expressing a decision made at the point of the conversation but rather intention and a premeditated (planned) event.