Why destroy Laketown's bridge?

[Sauron the White]

There is a far more obvious point.
Why would Smaug need to land at all?
He has no use for that bridge.
What is the point of destroying a bridge to the land if the dragon Smaug does not need to use that bridge?

And you saw they would not have wasted their time. Fact is they had precious little time according to the author. How fast can they destroy a bridge? Are we to believe that a pre industrial people can destroy a large bridge in the time it takes a dragon, already in sight, to fly to them?