Charles The reason to hesitate over playing . In some cases White might be aiming at the nose tesuji with . is then basic instinct, but White can fight back and the escape at a is still open.
Bill: Through White's bad shape with remains, if not worsens, since it now forms two empty triangles. The exchange, - , doesn't look too good, either. Black may continue with . Black aims at Ba.
(BTW, do the four Black stones including and form a farmer's hat? ;-))
Bill: If White lets Black live in the corner with , then the stretch, , is simple and strong. White is weak on both sides.