Long shadows can add great perspective to a slide.
Lucky Luke needs them to show off his speed. Photographers like
Heinrich Heidersberger have used them nicely in photo compositions (see the "Street Scene" image below)
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They are very easy to make in PowerPoint. I suggest forgetting about the built-in shadow functions of PowerPoint, they can be tricky control. Instead, draw your shadows using rectangular boxes.
Below a chart that can be used as a setting to display the 3 (or so) key messages of a presentation on a final slide (excuse the bullet points):