> An easing function takes in a linear progress value, and returns a new progress value, but converted to nonlinear motion.

They then describe the problem with them in the section "Easing is not so easy", after introducing easing functions that are already more expressive than `cubic-bezier`:

> In traditional animation, the principles are just guidelines; you still end up creating new unique motion each time based on what acting the scene calls for. Easing functions in code don't quite give you the flexibility to do that.