Based on the requirements, we estimate the cost and schedule it will take to deliver those. We then define a plan and work to stick to that plan. If something needs to slip, it will be the project cost or schedule.

In Agile, we flip the constraints upside down. Rather than constraining requirements, we instead constrain the cost and the schedule. Based on the provided cost and schedule, we then estimate the features that can be delivered. This means we follow a process that tries to adapt to change. You may not deliver all of the features, but you know that you will not overrun your cost, and can make the decision if you want to invest more money to continue the project.

Waterfall is a plan-driven model. Agile is a value-driven model. Which approach to choose depends on the situation and the ambitions. A hybrid approach is also possible, like building a house with waterfall and the interior with Agile.