Dry fowl pox and pecking injury are the usual causes, at least this time of year when it wouldn't be frostbite. Fowl pox is a virus carried by mosquitoes. It will suddenly go away on its own in about 3 weeks, and the chicken will then be immune for life. It doesn't cause them any trouble unless the lesions get a secondary bacterial infection, or unless they form in the mouth or throat, which is then called wet fowl pox and is unusual but much more serious.
The only reason to treat the lesions at all is to prevent secondary infection. You could dab them with a bit of dilute Betadine, or use an ointment like Neosporin (particularly near the eyes, as it will not irritate the eyes.)