By faith you will not be swept away utterly in the wrath of God. By faith you will live. And that is tremendously important for Paul's point in verse 16. Rescue from God's wrath and the gift of eternal life is given freely to those who trust God. We are saved from judgment and inherit life by trusting God.
Life is gained by faith.
The other is that it is a righteous person who gains his life by faith. Now that is crucial for what Paul is saying about the righteousness of God. But Habakkuk does link the righteous person and faith. And that is the other reason this text is so crucial for Paul's argument. If we have faith in him we will live and not perish.