If you watch Prof. Paul Hegarty’s iPad and iPhone Application Development (class #9 “Table Views” – link below), from Stanford, you will see that they ask him the very same question.
According to him, it is just considered to be “good practice” for the sender to send himself along the message, so that the receiver can message back in case it needs more info from the sender. He is talking about delegation specifically, but I imagine it is the same for target-action messaging like in this case.
So, it is apparently just a common design pattern that gets repeated over and over to account for all different case scenarios. In our particular case here, we will never message the sender back, so we can remove (id)sender completely if we want.
Here’s the link for that iTunes U video (free). The answer you are looking for is @ 1:06:00.