I believe there could be many hierarchy driven use cases for Dgraph. Directory -> subdirectory -> files or assembly -> subassembly -> machine part etc.
How could I traverse ('downwards '/ ‘upwards’) e.g a file system data and e.g. summarise total file size by file type extension to answer questions like: what are the 5 most space consuming file types in a given directory + subdirectories? How many files there are by file type?
The same analogy for machine parts could be e.g. a summary of weight or price or count by part type. In hierarchy each object would have exactly a one parent (expect a root), circular references would not be a problem.