One of the answers to a related question mentioned that Delta often puts a lot of passengers on the "awaiting seat assignment" list because these are Basic Economy passengers who only get a seat at the gate. I've personally faced this issue at least 3 times while flying with Delta.
But why is this the case? How does assigning seats so late benefit Delta's operations?
This happens when the flight is nearly full. Waiting to assign seats to people on Basic Economy fares allows frequent flyers booking last-minute fares and/or using Same-Day Confirmed or Same-Day Standby changes to the flight to get the better remaining seats, rather than being stuck in a middle seat in the back of the plane on a nearly-full flight.
On flights that are not near capacity, Basic Economy passengers will be able to choose any available normal economy seat when they check in (which they can do up to 24 hours in advance of the flight.)
Basically, Delta (like nearly all full-service airlines) wants to prioritize the experience of their high-revenue frequent flyers over that of infrequent leisure passengers on the cheapest fares. Also, the frequent business flyers are often operating on tighter schedules than the leisure passengers who are usually booking Basic Economy fares, so they try to do their best to keep seats closer to the front of the aircraft for those passengers.
I would say for operations it doesn't matter at all. The reason for Basic Economy fares was to offer the fare as cheap as possible, with additional fees for additional services. Seat reservation is one of those services. Many people booking BE probably don't care at all, but for those who care, the later they would get the seat assignment, the more likely it is they book a seat assignment for an additional fee (or specifically in Deltas case to book Main cabin instead of Basic Economy the next time).
