Birds feel safer on a roost. It keeps them away from ground predators and it is usually cooler in the summer.
Not sure where you live, but fire ants can sometimes be a problem if the birds sleep on the ground too.
The breed usually determines the height of the roost (and nest boxes). Heavier breeds need a lower roost because they can injure their feet and legs jumping down from the roost - 24 to 36 inches is a good height for them. Banties and lighter standard breeds do fine with a higher roost. Most chickens want the highest roost. If you have mixed breeds, I would keep the roost at a height for the heaviest bird and not offer options.
A dropping pit underneath is optional. If you can walk into your coop to rake out the muck, you probably don't need one.
The type of roost you use is also a matter of preference. In colder climates a wider roost (2x4 with flat side up) allows them to hunker down on top of their toes to keep them warm. That is what I use for all my roosts. You can go with something smaller or tree branches. Just make sure there are no sharp spots or splinters that can cause foot injuries. Round off the edges for comfort. Some people use round dowels, but research shows that chickens do not roost the same way as other birds and a flat surface is better for them.
