Why learn Java script/CSS/HTML?

Actually, all of those sites may let you edit some with HTML, CSS, and JavaScript.

Additionally, your exact needs may not fit the capabilities of various tools. As the saying go, "When you're a hammer, everything looks like a nail."

Therefore, let's say you wanted to make all outbound links have a special class added. Now, your site has existed for a while, and you can't find all of the links easily.

I could add about 3-4 short lines of JavaScript to a template file used by all of my pages that would allow me to (via JavaScript on page load) to add a class or another attribute to every outbound (different web site) link. For instance, I may force it to open those links in a new window and add the attribute rel="nofollow" so I don't lose any SEO benefits.

Additionally, sometimes these automated things have bugs. Knowing some coding allows you to make an immediate patch as you wait for a 3rd party to fix.

With WordPress, etc, you then depend on plugins from 3rd parties, or your platform's code. Well, as your business grows, maybe security becomes a big thing. You may be better served by building your own solution to ensure it meets all your legal and business requirements.

And finally, debugging. Let's say your site isn't working. The faster you identify the core issue, the faster it can be fixed.