Content templates are referenced by other templates for inclusion, but aren't surfaced in the Insert Template dialog or the New Page dialog. You can insert Content templates into the body of any page by using the Template button on the Editor toolbar.
You might use a Content template to insert a commonly used piece of information into the page you are working. The will eliminate the need to retype the content into each page. For example, you might create a Content template that contains a formatted note. You can use the template to insert the note into any page and then change the text of the note as appropriate.