Concierge service gives you a small gift in your cabin each night, special "turn down" service, and one person to call to book anything--Palo, spa, excursions, etc.
The concierge staff has a block of Palo and spa reservations just for cat 1,2,3 when you board. Any of those reservations that are not used on the first day are then made available to the rest of the guests. You will go to a meeting with the concierge staff shortly after boarding and make any reservations you wish.
Frankly, I don't think it's worth the extra $ unless you have a large family and need to bigger sleeping quarters. For instance, with 6 or 7 people, cat 1 and 2 become quite cost effective as you pay the high rate only for the first 2.
What the concierge does for you, you can very easily do for yourself with little/no hassle.