Bridges are miracles of architecture, human creativity. After all, there's a whole category dedicated to photographs of them here on dA. Every one of them is unique, fitted to the surrounding landscape, the available resources, and its supposed utilisation. They require most complex statical calculations, must be stable at strong winds, high temperatures and heavy load.
So when a bridge has stood firm for years, for summers and winters, with much and with little traffic, has thoroughly proved the quality of its construction, why just tear it down all of a sudden? Bridges are usually only destroyed at war, to hinder the hostile faction's advance. But that's always a costly thing - even a small or little-used bridge might be a great boon in the future. You can never have too many well-kept bridges, so to say.
But there the problem seems to be hidden - bridges, especially big ones, require a certain amount of maintenance, lest they crumble to pieces when the next lorry rolls over them. And if, due to external circumstances, a bridge becomes difficult to maintain, some might see it more efficient to destroy them, if they see no potential use for it in the future.
Again, I disagree - I prefer keeping bridges in all possible directions alive just in case I might need them sometime, even if that means an investment of work that seems wasteful at the time. But what good is that? For a bridge to stay standing, it requires the attention from the inhabitants on both of its sides.
And so, many bridges are destroyed or decay each day, as pointless as it is.