Management Consulting is basically to help clients in making important decisions. From critical perspective, some clients think of consultants as those who would borrow your watch to tell you time and then keep the watch (see this video). Others think of consultants from functional perspective, considering their advice genuinely useful. However, in either case, consultants reputation would be at stake if their recommendations do not make noticeable and positive difference in their client's business. Majority of consultancy work is based on referrals so there must be clients who should see them from functional perspective.
As introduced earlier, MC is about important decisions so let's see what types of decisions clients face. Although there can be many ways to categorise the types of decisions, we can divide them into two major types: strategic decisions and operational decisions. For example, strategic decisions would address questions like "How should we grow?", "How should we compete?", or "Should we acquire company X?". On the other hand, operations-level decisions would address questions like "Should we outsource this?" , "How to improve quality?", or "How to reduce costs?".
Why we need evidence-based decision making? To answer this, I would quote Daniel Kahneman who described its importance in a simple but effective manner: "I’ve been studying intuition for 45 years, and I’m no better than when I started, I make extreme predictions., I’m over-confident., I fall for every one of the biases." This is probably true for every human (and perhaps for machines as well) and there must be some evidence to support one action over another in order to avoid repentance later.