Why destroy Laketown's bridge?

[Rikae]

I think, Alfirin, that you're right in saying that spraying water on Smaug wouldn't put out his fire, but it certainly seems like total immersion in cold water would, at least temporarily, do so.

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.

(Assuming "quench him" means it would impair his fire-producing ability.)