Why destroy Laketown's bridge?

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Originally Posted by Sauron the White

It is wonderful that the human condition permits us to take the same set of facts and come to the very opposite conclusions.

It's that 'priors' thing, or which colour your spectacles are when you view said data.

And, not to add too much more, but aren't all aerial attacks followed up by a ground assault? Was Smaug just trying to stay on script? Did Sauron, who could have used Smaug if he were living at the time of the Fellowship, somehow want Smaug to preserve what he could of Laketown to use as a base?