Well, you do choose Javascript when you could have chosen PHP and had even more overlap. Presumably, because you believe it is better for the tasks. Similarly, Coffeescript authors believe that Coffeescript is better than Javascript -- even if there's less overlap.
I don't use JavaScript for anything for which I could have chosen PHP. Given that PHP runs in exactly 0 browsers, what do you mean? Few people are choosing server-side JavaScript over PHP, though it may happen occasionally.