Let's say you have some case law, and to know if it's relevant you need to pull the primary law/codes and regulations .You also need to know when the correct versions of everything are, so the case law needs to resolve to the correct version of the correct piece of legislation. "What is an atomic unit of legislation?" is also fuzzy as amendments will both move and edit named blocks of legislation at once. It's a big messy very hard to track pile of moving targets, and you need to account for the temporal aspect while linking documents not only to the changes but also to documents at certain revisions. Standard git and other version control has trouble expressing many of these concepts, and while it could be hacked in, I have found it more effective to create a purpose-built tool for the job.