Expressions in After Effects are a powerful tool for automating tasks and performing complex calculations.
You may want to use expressions in After Effects when you want to automate a task or perform a calculation that would be too complex or time-consuming to do manually. For example, you might use an expression to automatically change the value of a property over time, or to control the value of a property based on the value of another property. You might also use an expression to drive a complex animation or to link properties between layers.
In general, you should use expressions in After Effects whenever you want to save time on repetitive tasks or when you want to create complex animations that would be difficult to achieve using keyframes alone. By using expressions, you can streamline your workflow and create more dynamic and flexible animations.