Do you mean online check-in that you can do prior to arriving? The benefit is that you may receive a text or email when your room is ready which will allow you to bypass the front desk entirely. You can just go straight to your room. It also allows you to have your credit card already linked and pin already set to ensure an expedited check-in experience.
As far as physically checking in at the resort, you can check-in anytime after 6am on your arrival day, although your room may not be ready yet. The benefit of checking in early is that it gives you a better opportunity to receive a room that meets any requests you have. The CMs have a larger pool of room inventory to move things around in when you're checking in earlier in the day. If you check-in very late, the rooms left will be slim pickings if you want to request a room different from what you've been assigned for whatever reason.
