to not just ignore the tank and attack the other two guys. Without this threat and aggro system, it becomes highly illogical for the Video Game A.I. Often reliant on a simple "aggro" system, wherein the tank must keep the highest amount of "threat" in order to keep enemies focused on him - earlier games simply had foes go after whoever did the highest damage, rarely allowing a tank a place in combat, and earlier still featured them attacking mostly at random or just whoever they noticed first.
To compensate for their amazing restorative powers, they tend to be weak physically, but they may be able to hold their own quite decently (just not as well as the Tank, for game balance reasons). They may also have other support effects to help them - such as being able to put shields around allies, remove negative status effects or otherwise buff them, and sometimes even debuff enemies for good measure.