Pretty much all post release DLC gets made as part of balancing out workloads for teams as their requirements shift - when you’re in pre-production and early production you don’t need teams who can produced beautiful, polished assets, fancy skins, interesting effects, decorate levels, work out unique escalation combinations etc.
You do need the people who can work on levels, stories, etc at a conceptual level. You do need the people who write the complex dialog, invent new characters etc.
In the era prior to DLC it used to be a pretty standard practice that studios would either have projects going back to back or would hire large amounts of staff on temporary contracts, letting them go when their job is done. That was horrendous on the people working on the material and the on the end result, since it meant that smoothing out the bugs etc post release or trying make new content match the old was entirely dependant upon the employee being 1. available and 2. not so disgruntled they refuse to come back and 3. not so burned out they can’t work right now (or maybe ever)
Day One DLC was celebrated by many mid range developers because it let them manage these things better and afford to keep more staff on full time salary. Unfortunately many people who did not understand the relationship between studios and publishers ruined that.