These cover your cabin attendant and dining room staff. You still pay 15% on all bar orders, which is added to the drink price. If you order room service, you need to tip them for each order since they don't get a piece of the pre-paid tips. I always bring lots of dollar bills and will give $2 to $5 based on the size of the room service order.
You will see vouchers for the pre-paid tips delivered to your cabin on the last day. You put them in envelopes and hand them to the staff. They use the vouchers to get their money from the ship.