PowerPoint left pane with Slides and outline can be changed to show/hide it during the presentation design process. But if by mistake the pane appears hidden then you can restore it and show the left pane in powerpoint with this simple way.
Here is the version of powerpoint editor without the left pane. As you can see, there is a thin divider that once dragged to the right will make your left pane to appear again.
Hiding the left pane can be helpful if you need to hide the Slides to someone but of course showing again the slide left pane is also a great idea during the presentation design process since allows you to quickly move to any other slide.
powerpoint outline is also good to edit the presentation in the outline view. You can edit the presentation using this pane. Unfortunately there is no way to hide only one of these left pane and keep the other one.