A reduction in slugs, vine weevils and leather jacket-type bugs that live in the soil.
They'll scratch at your lawn to get at such things, but if you're around when they're out of their run they'll not do too much damage. They may peck at some greenery but mainly out of curiosity - hens are very inquisitive. Just keep them off seedbeds - little temporary fences of chicken wire and bamboo poles, or rather swisher stuff from here https://www.omlet.co.uk/shop/chicken_keeping/omlet_chicken_fencing/ that you can move about and place where you don't want them to go, will be fine.
I'm very envious - when I had the smallholding I had 60 rare breed poultry and odds & sods of bantams acquired somehow - I loved them - it was a toss-up between chickens or a pond in this garden, and the frogs don't mind if we go away on holiday
Oh, when they're fully grown get someone to show you how to clip one wing on each of them, then they won't fly over your fences
You have to re-do it each time they moult and grow new wing feathers.