the reasons I don't let mine totally roam
they eat my garden (I have a larger year around garden)
they poop on my porch and not on the boards i have to catch it so i can use it as fertilizer( I get about a 5 gallon bucket a day to use)
my neighbors think it's OK for them to have 5+ dogs each and let them roam to my house for chicken dinner and take a poop in my yard (regular rotation sss)
wouldn't get any eggs ( I sell most of them to help pay for feed,I use some and give some let some sit to make more chickens.)
they turn my
compost(I get leaves from different places and have a bagger mower they turn this into good stuff quickly )
so I get a lot from them for feed and yes they could do some of this without feed but then you loose your control on the out come
the reasons I would like to are probably the same as you .