1a.
• It lets UN send medical supplies that Damascus bans.
• UN acts as political cover for NGOs, which matters a lot to some of them and might be crucial in inducing Iraqi cooperation.
• NGOs lack UN's capacity to plan/coordinate.
• More of a humanitarian crisis could help IS respawn. It would also make al-Hol a bigger problem than it already is.
• Decline in aid from Iraq would force SDF to ask for it from Damascus, increasing Assad's influence in talks with SDF, undercutting US-Fra-UK strategy.
• Russia (and Turkey) wants SDF to submit to Assad, who controls aid from inside Syria. Assad's leverage increases the less aid SDF gets from outside.
• Russia views Yaroubiyeh as subsidizing U.S. occupation, suspects Trump will pull out if asked to cover the aid shortfall.
• Russia wants to normalize the UN/international standing of Assad's government.
• (But Russia doesn't seem to want the Turkey crossings gone, right now. So there's that.)
3.
Prestige. It seems to have become a game of chicken where both sides hope the other will back down.