The CORS standard is needed because it allows servers to specify not just who can access its assets, but also how the assets can be accessed.
Cross-origin requests are made using the standard HTTP request methods. Most servers will allow GET requests, meaning they will allow resources from external origins (say, a web page) to read their assets. HTTP requests methods like PATCH, PUT, or DELETE, however, may be denied to prevent malicious behavior. For many servers, this is intentional. For example, it is likely that server A does not want servers B, C, or D to edit or delete its assets.
With CORS, a server can specify who can access its assets and which HTTP request methods are allowed from external resources.