See T887. Some auth providers could theoretically support multiple instances on a single install: for example, you might have two different LDAP servers you want to authenticate against, or two different other Phabricator instances, or GitHub (public) and GitHub (enterprise), or multiple JIRA instances, or multiple MediaWiki instances.
These use cases are very rare, but there is no technical reason to prohibit them, and they would make some tasks (like //integrating// with multiple JIRA instances) easier.