For example, tables in relational databases like sqlite are representationally isomorphic to tabular files like the output ls -al. Sqlite has a lot of useful performance optimizations (including eg, not having to encode strings as "\x20" or the like if you want arbitrary bytes), but those come at the cost of a data format that you can't easily (not even notice that you needed to and did) reverse engineer when writing new software to consume or emit it, or even just visually inspecting it in a editor that doesn't already speak that format.
The main thing you can do with them though, is tell yourself that you don't really need a spec or documentation because you can just eyeball the output, and not having to follow a spec makes things feel simple.
You can do a surprising amount with flat files. For decades HP / HPE has been using flat files for Serviceguard / SGLX which is a high-availability cluster software that gets used in some very large Enterprise environments.