Why destroy Laketown's bridge?

[Sauron the White]

In THE HOBBIT, in the chapter Fire and Water, the people of Laketown are alerted to the coming of Smaug. They begin preparing for him and one of the things they do is throw down and destroy the bridge to the land?

Why would they do this? Smaug flies. He does not need a bridge to walk across the way some land based invaders would need it. Are not the people of Laketown cutting of their fastest way of their own retreat? Does Smaug need the bridge to land upon? If so there would be other areas of Laketown suitable for that purpose including some of the larger buildings. And then it brings up the questions of size regarding both the dragon and the bridge. In a pre Industrial society, how does a group of people quickly destroy a large bridge in what JRRT describes as "little time".

Could somebody explain this?