Problem: Suppose you have meta-data on some data sets and you want to select data for certain attributes. That sounds alot like a job for SQL. But the attributes are not strictly in a table format where you have something filled in for every attribute. You probably have not even decided (beforehand) what attributes you should have for every data set.
Solution: 'Convert' attribute (meta-)data into tables that SQL can query.
Note: The YAML part is just a convenience since it's expected that the meta-data is persistently stored. The meta-data abstraction is just one-level of nested dictionaries. I also hear a YAML reader can read JSON.