Props can stimulate an audience to pay closer attention because they are so rarely used. You may want to hold up a scale model of a proposed building that would be otherwise harder to visualize, or show a photograph of the building as it has been so far constructed, or a map with the route that Alexander the Great's army marched, or a bat that Babe Ruth used.
Props can make a big impact. For example, President Ronald Reagan once held up an enormous stack of paperwork about a proposed budget. He then theatrically dropped it to the floor and declared he would not approve such a budget.