Because systemd services can depend on mount points or cause mountpoints to come up dynamically, in a way that pure fstab does not support, e.g. for containers.

Because systemd services can depend on mount points or cause mountpoints to come up dynamically, in a way that pure fstab does not support, e.g. for containers. So (as far as I understand) fstab is now simply an input to the actual mount system.