Pre and Pro biotics are largely buzz words. Not they are meaningless or unimportant to us humans, but I think we can all agree there is a big difference between humans and their casual and widespread antibiotic use and frequently low-fiber diets here in America, and chickens. Chicken feed, lots of fiber! Little to no antibiotics. And while we humans do not share from one another's plates, chickens do all the time, with all the other things today ahem contribute ahem to the soil. So there are already many things in a chicken's environment which contribute to good bacteria in the gut, and help to maintain that, even when one chicken maybe having some small issues. If you happen to ferment your feed, you are also contributing to the prees and pros.
. Same if you should periodically give them yogurt as a treat.
Now, if for some reason you put your flock on high dosage long-term antibiotics oh, you might want to consider something to help get their gut back in order. Particularly if you noticed Oddities about the consistency of their droppings, or see any indications in body shape and behavior that their diet is suffering from inability to process what you are feeding them. But that's just being a good owner of chickens. Nothing magical about it. That said, like oyster shell it can't hurt. If It Makes You Happy do it.
And yes, I'm planting marigolds too, just as soon as it warms up some. Because we do eat with our eyes, and while my chicks yolks are already quite yellow from their free-range environment, I am all for adding additional variety. Can't hurt. And marigolds are pretty. Stupid reason, I know, but there it is. Probably something you never expected to read in one of my post...