Yes of course there are many other use cases. In my circle of friends, Joplin is used (and could be used even more) to take notes for research purposes. More and more researchers want to get rid of word processors and compatibility problems (libreoffice/MS). There is a tendency to open a Nextcloud for a research team (or with students) to share (and encrypt) data. Working in markdown allows to keep the objective of a reproducible research and to ensure interoperability. The txt formats are ideal for this (md, org, txt, etc). But many need to manage the complexity of writing an article, report or thesis in their markdown source files. In this area, Rmarkdown for example does an excellent job for graphics, data visualization. But everything works with a YAML header...
So yes we could use Rstudio or Zettlr as an external editor but in this case, we lose the benefit of the Joplin plugins which are very useful and more and more numerous.
This is a use case anyway.