The Scoorge of Dreamweaver

I see this CSS font stack declaration all the time font-family: Arial, Helvetica, sans-serif;. But what exactly is the point? In fact there is absolutely no point in declaring Arial, Helvetica, sans-serif as all Apple products have Arial installed. So no one — except maybe some UNIX user who happens to not have Arial installed but does happen to have Helvetica installed, or maybe some Mac user who has deinstalled Arial — will ever see Helvetica. The correct declaration should be Helvetica, Arial, sans-serif. So, why is this?