The international laws of war are generally much easier to apply in the context of a group of people claiming to be a state, than in the context of a group of people who do not claim to be a state.
For example, one of the critical questions for classifying an individual under the laws of war is whether the enemy combatant is publicly identified as a soldier of the entity against which a country has declared war.
In the context of an entity that claims to be a state, like Daesh, the distinction between identified soldiers, civilians, and covert combatants is relatively workable to apply.
There are actually tens of thousands of people who personally identify themselves as regular soldiers loyal to a particular Islamic State with a defined territory, chain of command and political leaders, and there are millions of people who personally identify themselves as civilian subjects of Daesh who are not soldiers, and there are probably even people who personally identify themselves as covert combatants for Daesh - like some of the European terror incident perpetrators we've seen over the last couple of years.
People applying the laws of war still need to do their best to accurately determine who fits in which category, but the categories in question are bona fide meaningful categories that are mutually socially agreed to by everyone involved that are reasonably well defined.
Recognition of something as a "state" for law of war purposes is also something quite different from recognition of something as a state for diplomatic purposes. Inherent in the laws of war is the understanding that one side may believe that the other side is an usurper or illegitimate government.
In contrast, in conflicts with many organized groups that are clearly non-state actors in the eyes of all involved, the distinction between publicly identified soldiers of a state and covert combatants is often ill defined.
Calling people citizens or subjects of that non-state group is likewise hard to analogize to the kinds of situations contemplated when the laws of war were devised. Does one have to be a "member" of the organization to qualify as a subject? Does one have to provide "material support" or conspire to carry out some specific acts? Or, does subjective support or a bare public statement of allegiance on a facebook page or bumper sticker suffice that did nothing material to aid the cause suffice?
This matters because, under the laws of war, subjects of an enemy can be interned humanely even if they are not combatants, and the lawful treatment of public and covert combatants is very different under the laws of war.