If you don’t use a child theme and need to insert some customizations, you can take advantage of the inbuilt editor and easily insert custom css and js code into the header or footer. Once code is saved, it immediately influences the workings of your site.
Since the code contained within the plugin is separate from your theme, it will be unaffected by the theme updating. If you do break something playing around, all you have to do is go back into the plugin interface and change a wrong item.
Also the plugin has a powerful filter editor for creating complex rules. They are convenient to use when you want to include your custom code for a specific url, post, page, device, os, browser, also it will be useful to execute js or css at a concrete date, time, day of week and etc.
Well, it's a good theme and plugin customizer and a perfect solution to add additional styles and js codes to your site.