What are hay balls used for with guinea pigs?

Ok, I haven’t owned any rodents for about 20years. But if i remember correctly their teeth never stop growing, so the reason you give them hay is so they can chew it to wear down the growth. Im guessing this hay ball contraption gets stuffed with ‘hay’, and hung in the enclosure for the guinea pigs.
?!?
Hope thats some help.

I can only answer in regards to rabbits.

Rabbits need to constantly chew on grass an hay. Digestion depends on the chewing. Timothy has is preferred because it does not make them obese.

I have two hay bins similar to your ball. The bunnies love them.