The word says it all: you go through everything one more time before you start your presentation. When you walk onto the stage, a lot of things will come your way.
You have to be able to concentrate fully on your story. Then you don’t have time to think about where to stand, how the sound works, whether the clicker of your presentation is there, etcetera. Check these things in advance, tune it with the technique and with the person who introduces you. This creates a lot of peace and quiet and ensures that you are not faced with any surprises.