Why do they put those fences around construction sites?

It is to stop small items, stones, sand dust and other debris from getting through.

Not a good idea, cuts off ventilation. Get some corrugated plastic roofing cut it into one foot sections. Punch three holes in the sides top middle bottom the same in each one, both edges, you can use a paper punch for this just wiggle it back and forth. Lay all the panels on the ground then thread a good quality polyethylene string that will fit through the holes twice. Start at one end and thread the string through the first hole and tie it leaving a long piece then loop it through the next hole twice to lock it in place use a short piece of 2X4 between the panels for spacing. Loop the string at every edge to lock them in place. Do this on both edges then tie the two ends together and make a big circle, i use my lawn mower to mulch and collect the leaves so i can get ten times as much leaves in the circle. When it's heaping full cover it with a piece of plastic to keep the rain and snow out.

I think it's so people don't stand around watching the workers all day plus the green stuff would help materials from going outside of the fence.

That's a good question.. Maybe to keep the dust and debris in. You know, from going through the fence into people's eyes and way and such.

why not hop the fence and ask them....?

maybe, just maybe....

they're trying to hide the real workers on their site...the "Oompa-Loompas!" Business has been slow at the chocolate factory lately because of everyone dieting... so they're expanding their handywork! LOL! =)

To stop the workers getting out

To hide all the tools they will have onsite, to stop site theft !!!

Some how they think the green mesh is going to stop people stealing things on sites....

To keep people from wandering in and getting injured.

And to prevent theft of construction materials.

Um I think it is because when people look to see whats going on they stop, do not pay attention and there is an accident.

To keep on-lookers from falling into the excavation!