In warcraft 3, you have player unit events and unit events. Player Unit Events are specific for all units of given player, while Unit Events are specific to given unit. Not every unit event will have player unit event counterpart. Unfortunarely, one of those that do not have player unit events is Unit - Specific Unit Takes Damage. This is very useful for stuff like custom bash, critical strike, damage amplifications and modifications and more.
However, it is not so simple to create leakless Player Unit Event from Unit Event, so there are those nice big complex systems that handle all the stuff for you