These days membrane covering is used for making flat roof. This helps in avoiding water to stand on roof.There are some more options also available for making flat roof but those are very expensive yet effective to serve purpose like metal, tin or copper material.Flat roof gives you option of terrace as well as utilization space on roof for future use and that is major reason why it is preferred so much.
In general flat roofs have no significant advantages but having flat roof can help you in making cheaper second floor on your already constructed house.These types of roofs are mostly common in Asian countries and the main reason is that those communities are used to such structures plus the population factor is yet another major reason to encourage such architecture.
I personally suggest that sliding roof is far more better than flat ones because the only disadvantage of that is some space is missed out other wise it has lots of benefits.