The tiered drive setup is more featured if you're interested in combining SSD+HDDs. Unlike other cache drive setups like ZFS's L2ARC/SLOG and even bcache, the SSD drives in this setup are still usable space. Otherwise I wouldn't want to use a 1TB SSD as a cache in front of a 4TB HDD for example.

Another use case would be on a handheld like Steam Deck: internal SSD tiered with a microSD for unused games I keep installed 'just in case'.

I would even want it on my home NAS. Instead of a separate root SSD and all my files on an HDD ZFS array (maybe with smaller cache SSDs), combine it all into a single bcachefs filesystems. Maybe with a subvolume root that stays pinned to the SSD(s). And also being able to use the full capabilities of different size drive unlike ZFS because it's a home NAS for cheap. No erasure coding yet though, so I'm in no rush to migrate my home NAS