Scripts that have been pre-registered using wp_register_script() do not need to be manually enqueued using wp_enqueue_script() if they are listed as a dependency of another script that is enqueued. WordPress will automatically include the registered script before it includes the enqueued script that lists the registered script’s handle as a dependency.

wp_register_script( $handle, $src, $deps, $ver, $in_footer );

• Registering scripts is technically not necessary, but highly recommended nonetheless.
• If the handle of a registered script is listed in the $deps array of dependencies of another script that is enqueued with wp_enqueue_script(), that script will be automatically loaded prior to loading the enqueued script. This greatly simplifies the process of ensuring that a script has all the dependencies it needs. See below for a simple example.
• So, the main purpose of the register functions is to allow you to simplify your code by removing the need to duplicate code if you enqueue the same script or style in more than one section of code. The benefits of this are many and probably don’t need to be listed here.

• The function should be called using the wp_enqueue_scripts or init action hook if you want to call it on the front-end of the site. To call it on the administration screens, use the admin_enqueue_scripts action hook. For the login screen, use the login_enqueue_scripts action hook. Calling it outside of an action hook can often lead to unexpected results and should be avoided.
• If attempt to register or enqueue an already registered handle with different parameters, the new parameters will be ignored. Instead, use wp_deregister_script() and register the script again with the new parameters.
• jQuery UI Effects is not included with the jquery-ui-core handle

By default, WordPress bundles many popular scripts commonly used by web developers besides the scripts used by core itself. Below is an (incomplete) list of the handles and paths of these scripts.