Vegetables provide much more vitamin C than can be provided by pellets. Plus, you can use them to bond with your guinea pigs, to the extent that they will see you as "the veggie person, wheek wheek wheek!"
Guinea Lynx :: Diet
Guinea pigs naturally eat some of their poops to ingest vital nutrients (mostly B vitamins) made in their cecum by symbiotic bacteria. This is called coprophagy.