Why Hide Pages?
This is so that you may test you page before you make it available for everyone to see. However, it does not make the page unviewable but simply hard to find because there is no link to it anywhere. Also, when you view a page, its link will appear in the sidebar navigation, but if you click on one of the pages at the same level in the navigation, you will notice that the link to the hidden page disappears.
As you can see in the picture below, there is a checkbox with the label "Hide Page from the Side-bar Navigation" at the bottom of the webpage editing form.
When a page is hidden it will show up in the CMS with parentheses around the title as pictured below.