Ducks and chickens have different needs and habits.
Ducks are inevitably wet while chickens are healthier when dry. Some people make it work.
One key thing is that if any of your ducks are male you MUST keep them away from the chicken hens because drakes have penises while roosters do not and...
It can be a space-saving measure -- extending a small run under the coop as well as beside it.
For some people it's easier to clean a coop that's arranged to shove a wheelbarrow under the door and scrape the bedding into it.
The major drawback is that if you can't reach in and touch the back...