Hiding pages can be immensely useful for many different purposes.

Creating test/staging pages

If you need to create a page to test how a feature works on your site, or if you want to keep editing and testing a page right up until you release it to the public, you can add them to the hidden menu. This will mean you can still easily view the page on the front end of the website, but users are very unlikely to come across it before you're willing to show it off.

Creating pages when you don't want to assign them to a visible menu (eg. the main menu)

Each of your menu items must be assigned to a menu. However, sometimes you may not want a page to appear within any of your accessible menus (ie. main menu, footer menu). In cases such as this you can add your page to the hidden menu, then just link to it elsewhere from an article, feature box, etc.

Removing pages from user's view without impacting your SEO

The more pages with useful information on your website, the more Google will trust your site and move it up the rankings. As such, deleting pages from your site can negatively impact your rankings. The best way to get around this is to move your pages to the hidden menu instead. This way they are not easily accessible by users, but Google can still reach them and decide your rankings accordingly.