https://calibre-ebook.com/dynamic/calibre-usage
The purpose of the beta is to increase the probability that there are no showstopper bugs in commonly used functionality on all the major platforms calibre supports.
Since calibre follows the release early and often development philosophy, and has a fairly extensive test suite (at least for more modern features), that is all that is needed.
Feature requests get taken care of in the usual calibre release cadence.