Why destroy Laketown's bridge?

[Sauron the White]

How does destroying the bridge to the land help the people of Laketown agains a flying fire breathing dragon who does not need that bridge in the least?

Nobody has answered that key question.

from Alatar

Of course, being a knighted defender of the faithful, I see no error here (or anywhere).

AHHHH!!!!! So this is a question of blind faith in ones deity. That makes the above responses much clearer.