Why is Barchu connected to communal Torah recitation? I get the pattern, in that we say it before the brachos of Shema and before the brachos of Kriyas Hatorah. But why? The intent behind it would seem to be that it's like a more universal call to prayer that should be just as appropriate for the Shema-less Mincha. The prayerful quality is especially pronounced in askenaz nuschaot for Yom Tov, especially Yamim Nara'im and the minhag of having aveilim lead the services just to say it (independent of kadish I believe). Have we just superimposed on it over the years? And either way, why specifically those words of blessing to introduce communal torah brachos?
EDIT: Msh, thank you for the welcome and help with clarification. Yes, I am essentially asking the related questions you say I am, and perhaps more, so I should certainly be more clear:
1) Am I correct in my understanding that Barchu was designed as an introduction to brachot over communal Torah recitation?
2) Assuming I am correct, why are those words chosen (i.e., "Bar'chu Et H' Ham'vorach: Baruch H' Ham'vorach L'olam Va'ed") for that endeavor?
3) Why does Barchu nonetheless appear to have the quality of a general call to prayer? (In addition to what I note above, I also realize that most non-ashkenaz nuschaot have it before Aleinu, but still only twice a day.)