I suppose this is identical in function to using an Action to define a field value when an entity is created, if a Formula is used. But “default value” is a simpler idea than “Action, triggered upon entity creation, etc”, and would be easier to maintain, since you can easily update a Field’s definition from the entity view.
It would also be useful to define Default Values via Formula for Text fields. Again, simpler to edit/maintain than an Action, since the Field definition can be edited directly from entity view.