Hi Fernman, I wondered the same thing about the bridge (whilst half way across it as a little threadbare) as you could just walk around it. I think it is older than the road beside it, looking at old maps there was just a footpath/track. There was a now long gone farm / outdoors centre just past it where the road now ends which might be why they put the road bridge in.
There have been highland cattle in this part of the Cambrians for quite a few years now, I think they suit the usually wet marshy and exposed environment, similar to parts of the Scottish Highlands I guess. Lovely to see them when out walking :-)