Some wikis, especially non-WMF ones, might have complementary user groups without clear distinction between users and administrators. So, one single user could have an account with "delete" permission, another with "editinterface" rights, etc., and use each of them for a different purpose.
Many bot-ops have multiple bot accounts for different tasks, too. Instead, Pywikibot only allows to configure one username per site.
The framework should also support CentralAuth (et similia) to automatically detect on which wikis a given account has bot/sysop/etc. rights.
T67196: Cannot be logged in as user and sysop at the same time