my preferred format for readable data is s-expressions.
more flexible than json, easier to write than yaml and easy to parse for a program. if you structure the data properly when writing it to the file, then it is easy to read too. yet it has the flexibility to losslessly map most programmable data structures.
This is a crucial question, actually - at a first glance, I would wager to answer:
"The Recutils format is not nearly as universal or flexible as YAML/JSON. It essentially supports just one data structure, analogous to a SQL table or a list of dicts. But for that use case (which is quite important) it does seem to a more suitable format, and provides additional features that those formats and their tooling do not."
So the set of advantages over json/yaml are similar to SQL's advantages, except the database format is human readable and grepable.