Something to do with soil erosion. They are putting up about 10 miles of them between the town where I live and the next town. The company doing the work is named "National Erosion Control, Inc."
Teknoid is right. When they tear up all the grass and stuff to build something, when it rained, all the soil would wash away, so they put that stuff up so the soil stays where it is.
Like others have said. Its to stop soil from washing away and making a mess. Same reason why you will sometimes see bails of straw in small stream, to keep silt from going down stream.
It has nothing to do with erosion. The regulations require that run-off from a jobsite does not increase during or after a project. Too much run-off will cause flooding and lower the water table. During construction, bales of hay and those low fences (as you called them) are used to control run-off. If you are building an office complex with a large parking lot, you will always see a retention basin or two on the property. This is to control run-off AFTER construction is complete. Basically, if the whole world was paved, a 1" rain storm would cause massive flooding. These measures control that to a degree.
