You might want to have a set of event properties that get included on all new events in your tracking plan by default.
Being able to mark an event property as default, would make it included by default on all new events. You would be able to remove the property if you don't want it on a particular event, but this could help you remember to include events that are supposed to be included on almost all events.
Default properties would be different from system properties in the way that developers would be prompted to provide it for every event when implementing the tracking code, while system properties are set once in the session and then automatically included on all events. That makes default properties a better fit for values that are dependent on the event that is being triggered, while system properties are designed for system/device/session dependent values.