Part of the conceptual switch to relativistic physics includes tossing the concept of a global coordinate frame which fixes every point in time and space uniquely. In order to toss this idea, while retaining a meaningful way to talk about causality, the concept of the event was introduced. An event is a place in timespace where something happens. You can trace the world lines of objects coming into the event, and going out of the event to other events, and in this way preserve causality (the order of events on any object's worldline) but leave time and space coordinates flexible for dilation. Also because different worldlines don't have any global coordinating clock, the dilemma of simultenaity doesn't rear it's ugly head.