Delta's MD88s and 757s are there purely due to the slowdown in the winter months, combined with schedule/capacity reductions as part of the BK reorganization.
There are also some lease birds that are parked while lease renogiations are underway.
Pending recovery of the airline, and schedule increases for spring/summer, those birds will be back.
It's also possible some may have been parked pending a major MX
event, and since the capacity isn't needed ATM, they've simply parked them 'til needed, and put off the MX
(and the resultant expense) ...
Now the 762s, on the other hand, those are destined for ABX
732s, probably, are destined for Coke cans.