The Custom CSS tab only allows you to edit the CSS for the element's main selector. It would be better if you could also access child selectors.
For example I'm creating a menu using the menu builder. I want to apply some custom CSS to the sub-menu items, but this isn't possible, because when you enter the Custom CSS tab for the menu element, it pre-selects the main menu selector and there's no way to change it. Of course, I can work around this by adding a global selector using Manage->Selectors, but it's not as organized, because then the CSS is no longer bound directly to the element. If I'm applying custom CSS to just one element in one menu, I want to do that in the menu element itself, not in the global selectors list.
In Elementor, you can apply custom CSS to an element by using a special "selector" selector. That selector acts like an alias to the element's main selector. The benefit is that it allows you to access not only the main selector, but any child selectors as well. So, for instance, if I wanted to access my sub-menu items in Elementor, I could do it like this: