Now they are back in the slot pool this means that airlines can apply for them now, and can factor them into any remaining S18 planning wiggle-room fleet and recruitment wise. Even a couple of weeks from now, this may be more difficult as airlines may need to source wet leases, 2nd hand aircraft or redeploy from other bases in order to utilise these slots - that takes time and if not done promptly could mean needing to cancel flights elsewhere in their network which are already on sale.
This is the answer - MAG's move was to get the slots released and available to allow proper planning by all concerned.