Chrome extensions exist in an “isolated world” to prevent global var collisions between the website and the extension that runs on a given website. This mostly applies to content_scripts in an extension.
E.g. window.foo: my extension declares foo in the global scope and the website the extension runs on also declares foo in the global scope. With “isolated world” scoping for the extension, this is not a worry since it doesn’t share scope with the web page it runs on.