Most elevated coops are small, built for just a few chickens and usually in people's backyards that have a space limit. You can get extra run space by having the coop built up off the ground about two feet so the chickens can go under it to get out of the sun or weather.
Some people build up their coops so they don't have to bend over to clean them out. They have a door in the side that they just open and rake/shovel out the bedding.
Built up coops also make it harder for some predators to get into the hen house.
It's totally a personal preference and may be determined by the amount of space you have to build a coop or the number of chickens you may have.