A properly tuned duroso set up will operate under a partial siphion. That is there is air going in, but a limited amount, so you get a sort of semi-siphion in the drain that gives you a bit more flow.
Any and all drains will slowly become obstructed over time (so you need to clean them). If you don't notice the flow slowing, it will eventually get to the point of backing up, and sooner or later might cause a flood. This can also happen fast if, for example, a hunk of cheato gets sucked onto the strainer. (Never run a drain without a strainer of some sort, if you do one snail can easily cause a VERY big mess)
By having the air inlet in a location where a rising water level will block the air, when the water does rise and blocks the air inlet, it will start the "full siphion". This will signifigantly increase the flow down the drain (preventing the flood), and will usually cause a very noticable sucking or flushing noise (natural alarm system - "HEY DUMMY, CLEAN ME!!!")
It's a very simple precaution that has prevented many many a flood.