Yeah, my experience is that... some of them actually do things, while others appear to be for show.
Sometimes it's even intelligent: at 7:00 pm with no one on the road, pushing the button will immediately turn the lights red (ok, yellow, heading to red) and give you a walk symbol.
Same location, 8:00 am as everyone is heading into work, pushing the button will make you wait exactly as long as not pushing the button until the scheduled changes happen.