My first thought as well! Even glancing through their website I don’t see an FAQ section or even a section just answering what value addition it brings over pandoc?
Jimmy uses Pandoc as backend. It is primarily meant for converting exports/backups from note-taking apps, like for example a "takeout.zip" from Google Keep. Supporting all formats that Pandoc itself supports is a nice side effect.
I will probably add a section how Jimmy compares to Pandoc, markitdown, obsidian-importer and others, though.