Feeding your guinea pigs hay has numerous advantages. Perhaps most importantly it helps them regulate digestion, ensuring that their food is processed properly. Without it, your cavies can develop stomach problems such as diarrhea.
Hay is quite abrasive and helps your pigs to keep their teeth ground down to a comfortable length. Unlike humans, guinea pigs' teeth grow constantly, and without a way to keep them short, they can grow too big for their mouth, making eating difficult and painful.