You may have photos in your camera roll that you would rather others not see, because they are simply private - whether they are embarrassing, compromising, have private data like passwords in them or otherwise. You probably want to keep them out of sight of somebody who might be browsing your camera roll, whether it is someone who stole your phone or simply a friend you handed it to to check out some pics. iOS offers a way to hide photos but surprisingly doesn't let you put them in a password protected album.
Just like with the Reminders app, you can drag and drop folders in the Notes app to group them together and form subfolders. You can then organize your notes inside your folders, create checklists, share folders with other iOS users to collaborate, and easily search for images, attached documents and more.
If you use Notes all the time then you know you can quickly become inundated with them and have a hard time finding important ones. You can make your Notes "sticky," like a post on a message thread that stays at the top, by "pinning" it. Open up your list of notes and simply swipe right on a note until you see a pin icon appear. Tap on it to pin the note to the top. Do the same thing to a pinned note to unpin it.
There are many ways to quickly access the Notes app. iPad owners with an Apple Pencil can simply touch the pencil to their screen to create a new note. While iPhone and iPod touch users don't have access to this trick, other options make it easy to write notes. iOS can even be configured to allow Notes access from the lock screen.
Yes you can use Face ID to open locked Notes on iPhone X. Head to Settings -> Notes -> Password and toggle it on.