When you are setting up your presentation room it's a good idea to actually sit in many of the chairs where your audience members will be sitting. You might even want to get someone to stand on stage and move around in the areas where you will be presenting.
You would be amazed at what they see. Your stage positioning may be blocking them from seeing visuals. Your visuals may not be as visible as you envisioned them when you created them. Maybe when you write on a flip chart you block off an entire segment of the audience which would tip you off to step away from the chart after you've finished writing down a particular bullet point.
Room support poles, video tripods, bad lighting, can all effect how a partcular audience member views your presentation. Checking each seat in advance gives you the knowledge you need to make appropriate adjustments so that each audience member can enjoy and learn from your presentation.