Why did people build covered bridges? Was the reason functional or purely aesthetic?

Functional - the roof protected the roadway and structural elements of the bridge from the weather - just like the roof on a house. These bridges were built when protection of wood from the elements was not well understood, and it was a lot easier to replace the roof and walls than the structural elements of the bridge.

One large reason they used to build covered bridges is because at the time, horses were crossing them. If a horse sees that it is walking on a bridge over a river it is likely to be "spooked" and bolt out of control. It was a safety feature based off of the traffic. Once transportation shifted to automobiles there was no need to cover the bridges.

My dad told me it was to protect the bridge from ice and snow so that it wouldn't weigh down the bridge and cause it to collapse. I'm not sure if this is the only reason, but it makes sense.

I think it was to prevent snow on the deck. Now a days, bridges are built with drainage in mind.

Both .