You should understand that the theme editor is used to change the code structure of the pages something that cannot be achieved by simply creating a page via dashboard.
These are two very different things, while you can add html on your page using the page editor you cannot add some Php there. In simple terms theme editor allows you to edit the structure of pages, styling and create new page templates as well.
To use theme editor to make changes you need to have some Php knowledge or CSS for styling.