Because you might want Google to index your website. Because you might want an integer or a float type. Because you might want to be able to test the logic of

Because you might want Google to index your website. Because you might want an integer or a float type. Because you might want to be able to test the logic of your code on one environment, the only environment where it will be run. Because you might want a reasonable dependency tree. Because you might not want to rewrite portions of your code to run them on the server side. I could go on.