Cost of capital, fundamentally, measures forward-looking risk. (Ideally, unavoidable risk.) A business that would succeed in a stable environment may not in a volatile one.
Set the rate too low and you waste resources. Set the rate too high and you pass on good opportunities. Hence society's interest in measuring this metric accurately.
From a microeconomic perspective, cutting lean when your competitors are buying market share is risky if the next few years will run smoothly. Likewise, burning cash while your competitors build balance sheet is risky if a recession is around the corner.