The only constant is change. Creating a six-month development plan and expecting the world to stand still while your team executes isn’t realistic. Agile allows you to adjust priorities and respond to changing business conditions, without sacrificing quality or team productivity.
When you are eight months into what was supposed to be a six-month project with no end in sight, that’s a real threat to your business. Agile forecasting techniques tell you when it will be done to within +/- 10%.
At the heart of agile is a certainty that you’re maintaining the product at a shippable level of quality, and you get to see progress every two weeks. When you or your customers review the latest version and decide it’s ready to go into production, you don’t have to wait until it’s “tested and hardened” for some unknown amount of time.