In its days as Al Qaeda in Iraq, ISIS was most associated with suicide bombings in Baghdad in particular, targeting Shiites and the hated Iraqi government. While some have argued that its return to these tactics is a sign of desperation after defeats on the battlefield, it's also about ISIS reminding Sunni Iraqis of why they supported it in the first place.
And that is not a minor detail.
ISIS is returning to its roots and it's also showing that even if it were to lose its caliphate, it would still be able to retain its original strategy and then climb back up again, suggesting that even in defeat, it would, like the Taliban, remain an ongoing threat.
Such losses would likely lose any outside recognition of its Islamic State and the pledges of allegiance it has received from outside terror groups.