Most creatures, great and small, carry worms and parasites. Yes; that includes YOU, too! If your chickens are fed a strick diet of commercial feed then you might want to deworm from time to time. Especially if they were raised when chicks isolated from their natural environment. Studies suggest that if they are free-ranged* from birth, they are much better equipt to handle the natural order of things that come their way.
*Meaning; fed a natural diet and in a more native/natural environment.
*Meaning; fed a natural diet and in a more native/natural environment.