forkrun complements things like SLURM (and even MPI). forkrun is intra-node, and is all about utilizing all the resources any given node as efficiently as possible, including when the node has a deep NUMA topology (e.g., it's EPYC-based). This allows SLURM and MPI to focus on inter-node work distribution and coordinating who gets to run things on which node and things like that.

tl;dr: forkrun takes over the "last mile" of actually running things on a given single node, so SLURM can focus on what it does best: efficiently allocating and distributing work to different nodes across the cluster.