[Plugin: WP Super Cache] so why are legacy wp-cache files generated at all?

Out of curiosity I am trying to understand why the legacy wp-cache files are generated at all when super-cache is running?

Super-cache is definitely working, I can see it creating new directories and files in them.

But wp-cache is also making it’s own html files in the cache directory.

Is it for logged in users perhaps?

Thanks for any insight into this before I start digging though WP code…

https://www.ads-software.com/extend/plugins/wp-super-cache/