These jobs are everywhere.
* vertical parking where a guy is there just to wave a flag and press a button
* private parking where another guy just tells cars exiting when to stop
* people handing out fliers and shouting advertisements
It's very clear for an outside observer that these are just filler jobs...
I'm not sure whether I'd call these jobs "useless" though. Sure, putting up warning signs would probably produce a similar safety level with lower costs, but somehow I feel better cared for if a human does the job.
(Which is not to say that a lot of these jobs decribed up-thread aren’t make-work, but this particular one probably isn’t.)
Alternatively you can send one person out to do that as nessisarily.
Also there are always people directing traffic in and out of a site, washing down every truck that leaves so there is no mud on the road, etc.
I would assume it is more a case of construction in a large city is more difficult logistically as there are stricter standards for noise, mess and pollution.
It was always my assumption that these type of jobs were on a rota. It would be pretty bad to make someone work the "human barrier/traffic light" job 52 weeks a year.
After living in a number of countries where construction works usually involve two or three people making loud mess for days I'm astonished how things work in Japan: a team of people shows up quietly, does the job efficiently and then disappears. You won't even notice them. There's a big-ish construction (they have heavy machines and dug up a big hole in the ground) going on nearby and I never heard any noise. And they're going with it really fast.