Or they could be there to protect historic buildings and prevent the downtown from becoming an eyesore.
There's no question there are people involved in getting historic preservation ordinances through who believe they are doing something to preserve their community. Visions of being the next Williamsburg dance in their heads. Unfortunately, it's a simple fact that every community cannot support itself on historic tourism. If there is no economic growth, or serious employers, no amount of ordinances are going to stop the decline of a town - it's part of the economic cycle. Towns can be preserved better by encouraging growth, allowing creative new uses of existing space, and by letting the invisible hand of the market show us what will make us prosperous.
In Europe, buildings that are 500-1000 years old made it to the 20th century without historic preservation ordinances. They were owned by people who chose to preserve them. That is the principal that should be used for preservation - people and groups that want to preserve building should raise the money to do so. The fact that they are unable to do so on any large scale says that it isn't as popular as they suggest. In order to implement their goals they end up relying on the power of government to do it for them, against the apparent wishes of people who've chosen not to spend their money that way.
Personally, the town I've seen be the most aggressive in this didn't exist at all in 1865. There aren't any truly historic buildings - they feel the need to preserve hundreds of Sears Homes. They say they're trying to maintain a family community in a college town, when in fact the college was there first, and the town grew in response to providing services to the college. They are chasing a vision of something that never existing, and are eager to use the power of government to build their perfect community.
That's where the power ends up, not with reasonable restrictions to preserve something everyone agrees on (like the community raising money to preserve a historic theater), but as a club to keep people in line, and make sure their friends get rewarded for the "right" kind of changes. These ordinances always end up being used on a far greater scale than originally itended, because power corrupts.