Essentially this is a way to lets you push all of the weird feature requests your customers might have off into one place.

From a customer point of view, maybe you need to rewrite the path based on the contents of a cookie, or maybe you want to shed certain types of requests in high load scenarios, or maybe there's a buggy upstream application that sends bad cache-control headers, or ...

If the goal is to let the customer specify infinitely complex logic at the edge, a programming language is a good way to do that. Function As A Service is a good billing model for lots of invocations of short, small functions across the customers choice of language.