Handle any reservations for trades to the non-DVC resorts, Adventures by Disney, Disney Cruise Line, the Conceirge Collections and trades through Interval International.
Help you with payments on your account or for your dues.
Take care of your dining and/or tour reservations for you. The regular time frames (180 days out for meals, etc.) apply.
You don't have to go through all of the questions like "Have you visited WDW at least 5 times in your life?" if you go through MS! My problem is, I often find myself calling for ADR's when MS is closed. I don't think there's any other benefit.
MS uses the same system to make dining reservations as the main dining line. Depending on location, members can reach MS using a toll free call. On the other side, the regular dining line is open earlier and later than MS. The regular dining line is also open on holidays.