why does a package from official repositories mentions what version of a dependency is required? Because it may be that it is working only with that particular version. That doesn't explain why it is needed or in any way useful for a package provided by official Arch repositories? Partial upgrades are unsupported [1]. Actually it could be vary annoying, if packages now start including the version of a dependency. I didn't notice that packages mention dependency versions for at least the last 4 years [2]. It's not the only dummy package I'm using for at least that long. No one said people are allowed to install only packages from official repos. You have yourself delivered an example of a package that had pulseaudio listed in its provides entry.
Another case: sometimes multiple packages in the official repo may deliver the same thing: see jre8-openjdk and jre7-openjdk — different versions of the java-runtime. Or mysql (*now* no longer in official repos) and mariadb: different implemenations of the same flavour of RDBMS.