Welcome to BYC Michelle! Glad you decided to join us.
Chickens lack stomach acid like humans, so they need grit to help aid in their digestion. The chickens need grit whenever they are fed any treats that needs chewing. I don't feed my chickens grit because they live in a sand/dirt run and have access to this the majority of the day. If your chickens don't have access to dirt or sand, and you are feeding them treats, then grit is needed.
Good Luck!