Why destroy Laketown's bridge?

[Sauron the White]

from skip spence

And please StW, don't ask me to explain how they managed to do so. I do not know how they did it, no one here knows. JRRT perhaps had a good idea in his head of how this was done, perhaps he didn't. All we know is what he wrote, and in The Hobbit the bridges of Lake Town were "cut" before Smaug could get there.

Well in truth Skip, we have a great deal more than "all we know is what he wrote". JRRT also gave us visuals of his world. In the case of Laketown, the picture that he himself drew clearly shows a rather substantial bridge with many thick pylons supporting it. It is clearly not a suspension bridge or something with ropes holding it up so that it can be quickly "cut".

That same drawing also evidences wide docks that are built along the same lines of the rest of the town. If Smaug wanted to land, and there is no evidence in the text or in drawings that was his intention, he had ample space to do so on the wide docks surrounding Laketown. But he decided not to.

So we do have much more than just what he wrote.