If you are unsure about the purpose of putting gravel on flat roofs, know that there are some really good reasons: to protect the underlying roof materials and to weigh down and secure them.
However, adding gravel to a flat roof must be done after a specialist has inspected the roof and made the necessary recommendation, because there are different types of gravels
In the case of a built-up roof (tar-and-gravel roof), you have to apply a layer of gravel over of the final coating of asphalt; the gravel will embed into it, which ensures that it will stay in place. Gravel prevents cracking, blistering and overall degradation, and the surface of the roof will have a longer lifespan. It also allows water to evaporate quicker, considering that it holds and releases heat constantly.
Gravel also ensures a better grip for those who deal with maintenance and repairs.
However, modified bitumen roofs (made from a mix of rubber and asphalt does not require additional gravel on top, because it already has a layer of gravel on the top layer.
When it comes to ingle-ply roofing systems (EPDM, TPO, PVC) specialists use ballast instead of gravel, because it weighs down the roofing materials, an provides similar protective benefits. Look to professionals for all your commercial roofing options at https://www.coloradosuperiorroofing.com/centennial-roofing-construction/.