Presentation notes
There are plenty of ways to create presentations that also offer a way to show notes (e. g. LibreOffice, Powerpoint). However, I've come across some situations where I couldn't use these notes. For instance, if the file format is required to be a pdf or if the place you are giving your presentation doesn't have a second monitor (and you aren't allowed to or can't connect your own device).
You could of course just write your notes on a piece of paper or put them in a file on your device. But having a seperate app would allow some neat features. For example, the App could listen to your presentation and dim the points you talked about in your notes and auto-scroll, if all visible points have been marked as talked about.