Explain why are capital market data and information useful when a firm is considering its cost of capital?

Capital market data and infomation is useful because while calculating the cost of equity , it has a component of beta and market risk premium which is affected by market risk since market return in affected by adverse news . and the beta is the measure of stock's volatility to the market and weights we use is also the market values and not the book value . So market data are useful while considering its cost of capital