Conventional Value at Risk analysis implicitly assumes that the future will resemble the past. Scenario Value at Risk lets you see how the Value at Risk picture changes under different future economic scenarios.

Scenario VaR can be used to perform stress testing. Since you are allowed to make any forecast that you want, you can subject your portfolio to stress tests to see how its VaR will change under extreme conditions (such as a market "crash," large changes in interest rates, etc.).