Your presentation is not complete-not perfect-until you add transition effects. Transition effects allow you to determine how long a slide remains on-screen, how each slide is placed during presentation (e.g., whether from left to right, from top to bottom, or diagonally across the screen), and which effects are used to display or introduce your slides (e.g., zigzag).
Once you have finished creating your slides for a presentation, adding transitions is easy and very useful.