When I was competing in speech in high school I remember one speech very clearly: The young man got up in front of the room, faced the chalk board with his back to us and gave his entire speech in that position.
I remember being shocked. He was in a competition. What was he doing?
I believe he was not looking at us to help him deal with his nerves.
But I can tell you another thing he was doing … he was making his speech very forgettable. I have no recollection what his topic was. All I remember was spending 10 to 15 minutes looking at the back of his suit.
Eye contact helps bring your audience into your presentation. It helps make them feel engaged and, for some, validated. It helps your audience root for you and the success of your presentation.