Basically, a WYSIWYG HTML editor has developed for helping newbies to create new and interesting websites easily as in the beginning; they do not assess sufficient knowledge about each and every part of HTML or website development technology.
So, having such HTML editor can be a great help for them to develop great websites effectively and successfully. Though, initially, WYSIWYG HTML editors have been come for newbies only, but with time, it is being used extensively by knowledgeable and experienced professionals as well.