Why destroy Laketown's bridge?

[Rikae]

Originally Posted by Sauron the White

While he was talking about landing on the docks, the same reasoning could have applied also Smaug being anywhere near water where he could misstep. That was my point which I think was a fair one.

Well, this is what you said:

Originally Posted by STW to Mac

If I may quote you "one mis-step and Smaug would end up in the water and his attack would have been foiled". That applies to the bridge also. Or do you think a thin railing perhaps three feet high would restrain him if he lost his balance?

Here, you ask Mac if he thinks the railing would restrain Smaug if he "lost his balance". Are you arguing that Smaug would be just as surefooted in the process of landing as he would while walking? Sure, that's possible - but the point is, it is also quite possible he wasn't, and the this makes more sense in the light of the text itself.

The bridge was gone, and his enemies were on an island in deep water too deep and dark and cool for his liking. If he plunged into it, a vapour and a steam would arise enough to cover all the land with a mist for
days; but the lake was mightier than he, it would quench him before he could pass through.

Ok, apparently it is significant (in Smaug's being foiled) that the bridge is gone, so clearly he had plans involving the bridge. What could those plans be?
"His foes were on an island in deep water" - clearly the water is a problem - which it would not be if he preferred to attack by air.
"if he plunged into it" - it appears he has a choice between his plan involving the bridge, risking "plunging into" the water, and what he subsequently does (attack by air).
"it would quench him before he could pass through". Now, there is no sense in saying this unless, were it otherwise, he would try to "pass through" - which means, if the water would not quench him, he would have attempted a land (or water) attack.

Now, this may not be the best possible interpretation of the passage, but I haven't heard a more plausible one proposed.