First niche is the full featured CoW filesystem; it has snapshots, detects and repairs corruption, transparent compression, all that good stuff.
The other niche is being an allocator of sectors. There's one storage device, divide it up amongst all these processes asking for storage. That's Ext4: an allocator of disk sectors, dressed up in a filesystem API. When you are running databases or VMs, all you want is an allocator of sectors. You don't want lots of stuff getting in the way of your writes. You don't want checksumming, you don't want your writes going to a new place every time. You just want write access to part of the disk.