Preening is a vital part of a chicken's grooming routine. By carefully preening their feathers, they can remove dirt, dust and parasites. This helps to keep their feathers in good condition and also helps to keep them warm.
Preening also helps to strengthen the bonds between feathers, which is important for maintaining the structural integrity of the chicken's plumage. Additionally, preening helps to distribute natural oils throughout the feathers, which helps to keep them water repellent.
While preening is primarily a means of keeping their feathers clean and healthy, it also serves as a social activity. Chickens will often preen each other, establishing social bonds and hierarchies within the flock.
So, in short, chickens preen themselves to keep their feathers clean and healthy, and also to socialise with their flock mates.