The purpose is to provide a way to sign URL without needing one of the provided Cloud Storage client libraries. Since it's a rest API, you can call it from any language you want that can make HTTP requests and perform RSA signatures. From that page you linked:
This page describes an algorithm for implementing the V4 signing process so that you can create Cloud Storage RSA key signed URLs in your own workflow, using a programming language of your choice.
The example shown is in python, but the text makes it clear that:
The sample uses the Python programming language, but does not use the Cloud Storage Client Libraries: