Explanation:

A deliberate look in the eyes of an audience member can communicate how much you care about their thoughts. Sustained eye contact is an invitation to turn your talk into a conversation. It creates a bond between speaker and listener, a connection that is beneficial to both parties. When you look someone in the eye, they are more likely to look at you, more likely to listen to you, and more likely to buy into your message.
Deliberately focussing your eyes on different audience members will help calm your nerves and clear your mind. Keep your eye contact steady so you can concentrate on your message.
People will be more willing to participate in the speech when they see you scanning the crowd. You’ll notice them nodding, frowning and even smiling. As a result, your audience are transformed from passive listeners to active participants.

If you don’t focus on different audience members or are looking at the floor (or your slides), the audience are less likely to engage with the presentation and start thinking about something completely different - you’ll have effectively lost that participant as they are no longer listening to what you are saying.