More pressure put on ISIL as Raqqa is also in the cross-hairs

Washington Post:

With the battle for Mosul underway, U.S. sets sights on ISIS capital of Raqqa

Though the campaign for Mosul is far from over, Defense Secretary Ashton Carter suggested the battle for the militants’ de facto capital in Syria could soon begin.

This would make it harder for ISIL to lash out in different directions as it comes under attack in two large locations.   ISIL's usual tactic while under attack is to have attacks in other directions to distract from the main attack against it.  In the early days of its operations, these attacks were successful in many cases expanding the caliphate control within the region.  With two attacks to deal with and a shrinking fighting force that tactic is unlikely to work this time.