I'm using images from
<http://maps.google.com/maps/api/staticmap>

and noticed that the links work without a parameter API\_Key e.g. :

<http://maps.google.com/maps/api/staticmap?center=53,51&maptype=satellite&zoom=17&size=640x300&sensor=false>

why should one use API\_Key parameter if it's optional? Is there any reason?