For enterprise valuation the discount rate is wacc; for equity cash flows it is cost-of-equity. Companies also use an internal hurdle rate for project approval, usually set above WACC to leave a margin for error.
A one-point change in the rate can move a valuation by 15% or more, which is why the rate deserves as much thought as the forecast. It is also the channel through which central bank policy reaches equity prices.
Example: Northwind Tools valued at 8.1% gives $2.9B of enterprise value. At 9.1% it gives $2.5B, and at 7.1% it gives $3.5B, on identical cash flow forecasts.
Related: wacc, cost-of-equity, present-value, sensitivity-analysis, discounted-cash-flow