Why destroy Laketown's bridge?

[Rikae]

Originally Posted by Sauron the White

His speed and agility and overhead advantage is removed and negated. Its like getting a fast moving boxer cornered in a smaller ring where he cannot move around and use his advantages.

I don't know where you get the idea that Smaug's advantages (or even his greatest advantages) are speed and agility. What about his size and strength (knocking down roofs with his tail, crashing through the city), his fiery breath and his armour? If you want to fight him at close range, be my guest - I'd prefer to keep my distance.

It also lessens the distance between the fired arrows and means they have more force when they hit since they are not going so far out and upwards.

Indeed, they are not going upwards, and therefore are aimed at the toughest part of the dragon.

I would think the physics of war would be more on the sides of the townies when they can shoot at closer range and perhaps use buildings as shields and protection. But thats just speculation.

They can use buildings more effectively for this purpose when Smaug isn't close enough to smash or incinerate said buildings.