There are many reasons why you may want to use expressions (aka javascript) to power animations in After Effects.
Efficiency
Typing an expression is often much faster for automating animation. This allows us to tell After Effects what motion we want to automate, while working on other aspects of our design/video. These functions save a lot of time and allow us to elevate our work as motion designers, whether it's using the loopOut function to cycle through a few keyframes, or more complex code controlling hundreds of layers.
Templatability
I've often found it's easier for a user unfamiliar with After Effects to use a template built with expressions, rather than keyframes. Clients can accidentally move keyframes, even with layers hidden away, but very few people feel confident enough to mess with a block of code. By combining this with the "Essential Graphics" panel, clients don't need to change any information in the timeline or properties panels. Instead, they can find all the information they need for a template in one place, and can feel confident they are unable to accidently break the template.
Expressions can also empower our templates to do more. By applying expressions to parameters when creating .mogrt files for use in Premiere, we can enable much simpler templates for editors, able to introduce elements like drop down menus, sliders, and checkbox controls.
Connectivity
Expressions give us a way to connect After Effects animations and its render engine with other files and software. The simpliest example of this is with .csv and .jsx files.
After Effects allows us to import these file types into our projects. Expressions allow us to pull information from these sources, and use them to power our animations. This can lead to dynamic projects, capable of being changed as a spreadsheet updates, or allows us to store our own code in a .jsx file.
When combined with open source software like nex render, we can automate rendering workflows, and create data driven templates without opening After Effects.
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