There are special member-only encounters, or "instances," available for members. These instances are generally longer and more rewarding than free instances, and often have many member-only repeatable quests that will net you more stars, experience, and coins.
Along with exclusive member-only instances, there are also tons of member-only quests. While some of these are tied to member-only jobs (see below), many are independent quest lines. In addition, many free classes - such as the ninja - can only finish their final quest line as a member.