Tracing filesystem access is useful to figure out what a certain part of a program does, and taking actions based on that, such as creating a file dependency graph.
Tracing filesystem access can only be done when the filesystem is not globally available. Otherwise, filesystem access tracing cannot be done reliably when multiple parts of a program access the filesystem in parallel.
Making the filesystem not globally available, and instead passing in the filesystem as a reference, would make this possible.
For Nanoc, this is problematic because some filters do their own filesystem access, which Nanoc cannot trace, and therefore Nanoc cannot build the right dependencies.
The tup build system uses FUSE to spin up a filesystem, which is globally available, but allows tracing because tup is fully responsible for the filesystem.