Shopping is a time consuming process and users cannot always make quick decisions. While they need time to decide on buying an item, they don't want to lose track of it - 'out of sight, out of mind'. Introducing a wishlist helps users save their favorite items and keeps them in sight, gently reminding them to revisit.
Shopping alone can be a lonely experience, and while CRED encourages users to refer their friends, it doesn't have any facilities for friends to do together once a friend is onboarded on to CRED. Using shared wishlists, friends can interact with each other, shop together, comment and upvote each other's shopping items.