A tool is whatever you need it to be. My dad used to hammer in nails with a pestle for years until we got multiple hammers and insisted he use it.
While traditionally, announcements were made over the blog, it's a bit of a clunky tool at the current point, and to a big extent is more aimed at an external audience than the core user base.
On the other hand, there's this old, almost forgotten diagram:
which...which once resided in the abouttour page - and that really does reflect meta well - least for many of us, it acts as a community built repository of knowledge (aka a wiki). It's a place for announcements, and acts almost as a secondary blog. While I'd bristle at calling it a forum in the modern sense - it is a place for discussion. Not sure about these diggDigg/refitReddit things.
In a sense, folks at SE found that the 20 lb-lb sledgehammer worked less well for their needs here than one of these:
By Agolz12 - Own work, CC BY-SA 4.0
It's a multi purpose-purpose tool that does many things. It might not be the best tool for driving in stakes but... that's not what we always need.
So meta is used by Stack Overflow in this way since it's the most efficient way to get information through to the broader Stack Exchange Userbase.