Often it is as simple as finding the relative position that you would like to insert an element. It can also accomplish complex tasks such as editing the size and function of a particular element on the page. The freedom of the WYSIWYG editor allows you to play it safe by just working with text or the advanced functions of editing the source directly.
Any page on your website that uses the WYSIWYG editor can have the source code pulled up to work directly within it. This includes your main home page, content and area pages, blog entries and even custom content boxes in your sidebar or footer.