Going back as far as the Roman times, in ancient Rome Man has started using stone to protect drain covers.commercial concrete furniture for drain cover (grate) is manhole cover. Say "manhole cover" and images of spies/heros making use of the underground sewage system popping out of manhole cover in Hollywood films appear.
Historically from the time that Man invented the sewage system since the Roman Times to drain water and urban waste away for sanitation purposes, manhole covers (drain covers or grates as we prefer to call them) have been invented out from the necessity to avoid people from falling into underground sewage and also prevent large objects from entering the drains which cause choking which eventually may lead to flooding.
Fast forward to the current day, the sewage system is still employed under deck grates . As opposed to crude stone manhole covers the Romans used in the ancient days, we now have special steel reinforced stone gratings that serve the thousand year old function of covering the inlets of our sewage system.
Far more than a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates