The responses provided so far resonate with me - I will try not to repeat but to add on.
In addition to Q&A connecting experts with more junior people it is also about connecting experts across many content and skill domains. It is simply impossible in most situations for one person to know everything they need to know - even if they are a 40 year veteran. In most situations the world is changing and getting more complex. Connecting with each other via Q&A and discussion allows us to collectively continue to learn from each other.
In most cases I do not believe a social learning approach (Discussion, Q&A...) can fully replace a formal learning (training) approach. There is a time and a place for both and we are also seeing more and more examples of blending the two together. Formal learning provides an excellent foundation to build on - though few would argue that any training will get someone to deep expertise. It is thru on-the-job experiences and networking with each other that we can develop our deeper skills.
The world is filled with plenty of examples of experts making inappropriate decisions - in many cases because they didn't connect with each other (heard a great example last week related to the crash of a stealth bomber) or we connected to each other but didn't listen (Challenger) - or didn't have the right culture in place.
Improving Q&A/discussion at Accenture is one of my top priorities. I want to make it as easy as possible for any of our 200,000+ people to post a question and then be either directed to similar past questions/answers or direct the question to the right people or communities.