One example that comes to mind is containers. Since they run in user space they don't manage block devices themselves and use whatever filesystem the OS present

One example that comes to mind is containers. Since they run in user space they don't manage block devices themselves and use whatever filesystem the OS present them. Now how useful that would be ? I remember that with Solaris back in the days, using Sybase inside zones (containers) was always up for debate because database teams liked block devices for Sybase, for performance reasons (instead of ZFS for example), which was really annoying to manage and automate for Ops teams. I don't know how many stuffs nowadays behave better on block devices than on regular filesystems. Maybe it could be useful for filesystem development but that would be pretty niche and non-production.