An Event Context allows specification of an event which is needed to activate its profile e.g. SMS received, screen has gone off.
Events are a little different to other contexts because they are instantaneous whereas other contexts usually have a duration.
This means that it is nonsensical to specify that e.g. the screen brightness should be set to X for the duration of the event, so Tasker assumes that all settings actions should persist beyond the event.
For more information about specifying events, see the Event Edit screen.