I didn't the first time around. My coop sat right on the ground.
The second time around I figured out two things...
1.) The birds need every bit of ground they can get their little feet on during the day. So, the ground under the coop made a great addition to the existing run area. Along that same thought, the extra area was covered and made a SWEET area to put feed and keep it dry.
2.) I learned that the birds would not get bumble foot if I raised the floor so that the birds did not have to jump so far off the roost. Now, I could of lowered the roosts, but then I would of had to of lowered the nesting boxes too OR the birds would of started sleeping in them at night. Seriously, I like the nesting boxes up high so that I don't have to bend over to get the eggs.