Maybe it is to prevent the peeled nanosheets from reuniting! Early theories predicted that a completely flat single layer of graphene would not be stable. Although single-layer graphene was later experimentally prepared, and research showed that the surface of graphene was not completely flat, but slightly undulating (conducive to the stable existence of single-layer graphene); but completely free (especially with larger areas) ) Graphene has not been observed so far, and is often replaced by graphene on (or relying on) a substrate, or graphene dispersed in a solution. There are various indications that perhaps the stable existence of single-layer graphene is due to the interaction between graphene and the substrate or solution. This may help you understand that exfoliated graphene needs to be dispersed in DMF.