It's pretty common in Asia and Eastern Europe, maybe other places as well. It's usually more of a half day. As for why... probably because Saturdays are work days for adults too. The 5 day work week seems to be gaining ground though.
Students had to go to school on Saturdays in Sweden too, but that was later abolished. It was always test day, my father have said. Even then, it was only really half a school day, since they were allowed home after lunch.