Companion to the mountains piece. The question is the same; the angle is different - we focus here on what flat terrain means for the map's design.
For the underlying geography, see Leonida vs real Florida.
Verticality has to come from buildings
If the natural terrain is flat, the high points have to be built. Vice City's skyline gives the map its tallest points. Bridges and causeways give it crossings. There aren't natural lookouts.
This is similar to how Liberty City worked in GTA IV - a flat, dense urban environment where vertical play came from rooftops and architecture rather than from terrain.
What flat terrain enables
Long sight lines. Highway driving that feels open. Wetlands traversal where the horizon goes on. The bridge routes feel longer because there's no terrain to hide behind.
Rural gameplay also reads differently - no hills to roll over, more saw grass and orange grove and flat road.