That would obviously be ideal but announcing after the feature is implemented may be too late.
Opinion shifts have a lot of momentum - let's say promising a feature pushes back on that shift with force X. Announcing the feature when it's live and ready to use pushes back on that shift with force Y. Even if Y has much greater magnitude than X, it is likely much more effective to reverse that opinion shift by applying X today, than Y in a week, or two, or four, when the shift has gathered sufficient momentum that Y has no effect.
Opinion shifts have a lot of momentum - let's say promising a feature pushes back on that shift with force X. Announcing the feature when it's live and ready to use pushes back on that shift with force Y. Even if Y has much greater magnitude than X, it is likely much more effective to reverse that opinion shift by applying X today, than Y in a week, or two, or four, when the shift has gathered sufficient momentum that Y has no effect.