You will find a whole lot of us don’t have our coops raised. There’s probably more variety in what we do for coops that anything else with chickens. I suspect what we do for nests would come in second in variety. We all have unique situations and we do different things. It’s extremely rare that with chickens certain things like this are necessary. A tremendous amount of what people think is necessary is really just personal preference. Just read through a few threads and you’ll see we have a lot of different opinions on about everything. That's because a whole lot of different things work.
A coop and run needs to stay dry. If you are building your coop in a low spot where water could run into it, then raising it might be a good idea. That’s my first thought on where to put the coop and run, what does the rainwater runoff look like. Avoid low spots.
If space is tight, raising the coop enough so the chickens can get under it is a way to increase the available space. Someone also mentioned it is a shady spot, which is important on the heat of summer. It can also provide a place to put your feeder and keep the feed dry.
If you raise it, you need to raise it enough so you can get under there, maybe to retrieve eggs or maybe a chicken that doesn’t want to be caught. If you don’t raise it enough so the chickens can get under there, you create a great place for a mouse or rat to raise a family or maybe something bigger to live.
How you manage your bedding makes a difference too. If you are cleaning your bedding out real often, a raised coop might be a benefit. If you use the deep litter method or some variation of that, it helps if the floor of your coop is dirt.
I think a coop on the ground gets some benefit from a climate viewpoint. In the summer, the ground may act as a heat sink to help get rid of excess heat. In winter, it provides a thermal mass. I think these help reduce wild temperature changes inside the coop.
I built my coop by closing off the end of an existing shed. I put a few inches of clay dirt inside to raise the floor and put in a swale on the uphill side to divert water away. Then I use wood shavings on top of that. I cleaned my coop out for the first time in four years last November, not because I needed to but because I wanted that stuff on my garden. I also use a droppings board to capture the poop they make at night on the roost for my compost. Some people clean their coops out weekly. We all do things differently.
Since you are building a new coop, I’ll give you some articles that I think should be required reading on ventilation and mud. I don’t know if the cold weather one applies to you or not.
Good luck!!!
Pat’s Big Ol' Ventilation Page
https://www.backyardchickens.com/web/viewblog.php?id=1642-VENTILATION
Pat’s Cold Coop (winter design) page:
https://www.backyardchickens.com/web/viewblog.php?id=1642-winter-coop-temperatures
Pat’s Big Ol' Mud Page (fixing muddy runs):
https://www.backyardchickens.com/web/viewblog.php?id=1642-fix-a-muddy-run
