Why a printed save the date is a one-time thing
A paper save the date announces a single fact and then goes quiet. It holds no details because there are none yet, and it cannot gain them later, so the card on the fridge says nothing new when the venues finally land. Guests who meant to respond have lost the card, and you start the whole invitation again from scratch months on.
An online save the date is the same early announcement with a pulse. It states the date and the town, invites a low-effort signal that someone intends to come, and then keeps its door open. When the ceremony place is confirmed or the plan fills out, you add it to the same page, and everyone who already has the link simply sees more.