Whenever you insert a new shape into a PowerPoint slide, it is filled with a solid color by default (or the default fill may be different depending on the Theme applied to your presentation). Most of the time, you may leave that unaltered, but there may be times you want just the outline to be visible. This sort of outlining focuses the part of the picture that has to be highlighted. It is in scenarios like these where it can be useful to learn how to remove any fill.
Remove fill from shapes in PowerPoint 2016 for Mac.