Puppies explore with their mouths. Everything and anything will go in. When they begin teething, chewing eases discomfort and helps puppy teeth work their way out.
Adults chew for fun, to alleviate boredom, because it feels good.
Chewing is natural and to be expected. Your job it to teach your dog to chew only appropriate items. Make certain you use items approved for dog use. Make certain the items you give your puppy or dog to chew are sturdy and will not be ingested rapidly. They can be eaten fast, offer no nutritional value and do not give a long lasting, satisfying chew.