If ISIS moves on Sawran and then captured Azaz, it will cut rebels off from a critical supply line into Turkey via Bab al-Salamah and could arguably spell the end for an effective opposition in Aleppo.
Losing Marea to ISIS would prove a crippling blow for Liwa al-Tawhid, whose founder and former leader Abdulqader Saleh was from the town. It would likely enable ISIS to continue towards Anadan and Hreitan, immediately north of opposition-controlled districts of Aleppo city, and cut off any remaining rebel forces there.
Eventually, this would also open a route towards the former ISIS towns of Darat Izza and Al-Dana to the west in Idlib governorate, where ISIS could threaten what would then be the last remaining border crossing with Turkey at Bab al-Hawa.
Late on Wednesday, while mosques in Marea issued calls for a general mobilization to defend against the expected ISIS assault, civilians and rebels stockpiled food and supplies in the case of a siege, according to people I spoke to in the town.