I mean, it's happening whether you're using snapshots or not; ZFS doesn't overwrite things in place basically ever. Snapshots just mean it doesn't delete the old copy as having nothing referencing it.
CoW is always happening but without snapshots you can know that every block has exactly zero or one references and it's much simpler. No garbage collection, no complicated data structures, all you need is a single tree and a queue of dead blocks.
