Why destroy Laketown's bridge?

[Sauron the White]

When I post these questions it is because others have more knowledge than I do about many of these things. Often, it is quickly resolved and I see what I missed in a mere simple reading. But in this case, it seems that many people who see it differently - and that is certainly their right - are basing much of this on a series of assumptions - any one of which can be challenged and doubt cast upon it.

In this case, we have the assumptions that

*** Smaug was intending to land on the bridge and walk into Laketown

*** he needed a airport sized runway in which to land and only the bridge afforded him that

*** while the bridge was big enough, no other structure in Laketown was including the docks area

*** while the city was crowded and cramped not allowing him to land, it was big enough for him to manuever around - or perhaps small enough enabling him to walk over the structures

*** the bridge was built in a special way so that common folk without power tools or explosives could not only down it, but actually destroy it in the brief time it took from the sighting of Smaug to the arrival of Smaug over Laketown

*** A flying, fire breathing dragon would decide that he was more devestating upon the ground, moving slowly and deliberatly in a crowded town than swooping fast and lithe from above

Okay. These assumptions may be accepted by many here and that is fine. I just cannot in good use of my faculties accept them.

Maybe I simply do not have the faith.