Most STARs are indeed within the FMS Database; however, there are different levels of navigational accuracy that can be achieved by an FMS. An FMS STAR (with waypoint LNAV and VNAV sequencing) might typically have several steering waypoints between two identified waypoints within the STAR; hence achieving a much more accurate approach path which might otherwise not provide an accurate autopilot coupled track between two single waypoints. Essentially, this provides more accurate track keeping; particularly within noise sensitive areas. Hope this helps?