Cliffs are part of the natural terrain generation. Cliffs are usually generated in lines with gaps, creating significant obstacles that have to be avoided until they can be blown up using Cliff explosives. They do not drop any resources when removed. Driving into cliffs does not damage the vehicle.
Cliffs are not generated in the starting area. While it looks like they create elevation, this is just a visual trick and no elevation is actually present. This means that other entites can be built around cliffs as usual, including underground belts/pipes going "under" cliffs.
Turrets can be placed "atop" cliffs for a tactical advantage - firing from an area the enemy can't easily get to. Players should be wary of the environment, as it's possible to be cornered in an area surrounded by cliffs; however, players can also use such a setup to funnel enemies into a specific area.