It orchestrates you and your customers' code by allowing them to run custom code after specific actions. For example, HackerNews could implement this and allows us to run any code right after we post a comment, so we could send an email, call an api, perform a database call, etc.

What's the benefit? You don't need to set this up yourself (security, authentication, validate tokens, create endpoints).

Why don't create an API so we have more control rather than give customers the ability to run any code? Well, it depends. If you can keep up with customer requests, then go ahead with an API. If you have a lot of requests and it's becoming hard to keep up, then you can use Workers for Platforms.