What Dukat said. Raqqah has long been abandoned as a main city- they've left behind enough guys (estimates ~2500-4000) to effectively turn the whole thing into a long, brutal battle, but they pulled all remaining resources out. They're throwing everything they can at Deir Ez Zor because, if they can finally crush it fully, they would be able to create a box of a last stand around it, Mayadin, al-Bukamal, and al-Qa'im. Right now, they know it's gonna end for their little run, but they want to make it last as long as possible.
That said, there are reports now that they're arming Raqqah citizens and forcing them to fight the SDF (most likely interspersing them amoungst actual ISIL forces and holding families hostage to force them to obey), so that'll be a shitshow. Good chance they'll use them as forced suicide bombers as well (blow yourself up or we torture your family to death, etc).