Skilled presenters use silence to add to the effectiveness and polish of a program.
Theatrical folks have identified a whole bunch of neat pauses which I'm sure they have a ball playing with. I'm only going to address some of the most obvious and important ones here. The shortest pauses, which last anywhere from one-half to two seconds, are for the simple purpose of separating your thoughts. All you have to remember is to slow down. Give the audience a fighting chance to absorb what you are saying.