Why (id) sender instead of sender (UIButton *)

I’m trying to learn about Objective-C for iPhone development, and while looking at action examples, I often saw things like:

- (IBAction)sliderMoved:(id)sender
{
    UISlider *aSlider = (UISlider *)sender;
    //rest of code goes here...
}

What is the purpose of entering the type (id) and then casting it immediately (UISlider *). Why not just do the following:

- (IBAction)sliderMoved:(UISlider *)aSlider;
{
    //rest of code goes here...
}

Is there a technical reason for this, or is it just an agreement or made for readability?

Thanks in advance,

• Timo