Sforno on Exodus 17:6/ספורנו על שמות י״ז:ו׳

1והכית בצור ושתה העם, this will teach them that the staff is not an instrument which performs certain functions naturally, being unable to change its functions. It will show you that the staff is an instrument carrying out the will of its owner. This is why it can be used to perform opposing functions on different occasions.