Why is it important to know HTML when learning web design (even though we will use a WYSIWYG editor like Dreamweaver)?

We use HTML to create pages and site. We use it in notepad. It takes some practice to master it. It is very easy and fun. Even though it is much easier with DreamWeaver creating pages from notepad is more fun. Mark Zukerberg created The FaceBook from a similar thing like notepad.