We use shredded paper for bedding. It's free I think, I'm not resonsible for the supply and currently can't justify the expense of supplying an alternative.
Habit. The tribe coops in Catalonia didn't have bedding, except for the nest boxes, not did my uncles coops. Everybody roosted on bars.
We don't get a consistant size of shredded paper; some fat and long, some thin and short. The thin and short is better. I clean the coop every day and take out the bedding with poop on. 15 hens is a lot of poop. We get through a lot of paper.
The pastic floor is easy to clean with a paint scraper and I get a better idea of the quality of the poop. It's hard to tell in the paper mainly because when I get to the coop the hens have kicked a lot of it around.
I am more likely to clean the floor properly, sweep and scrub out if there isn't bedding to get out first.
The sweeping gets a lot of the dander and dust out with is healthier and a damp brush dipped in disinfectant and water helps to discourage mites. It doesn't need to be soaked every time but even a damp brush helps.