I'm not a dev so I dont know how anything works, I barely know how to use tools that I play with.
I dont fully understand source mapping and I've never used it, but from what I've read, if source mapping is implemented you can use it to download the source files for debugging, but how can you debug if there is no way to make your changes visible in the browser? Unless I am completely misunderstanding what source mapping is supposed to do, which wouldnt surprise me.
Here is the docuentation overrides in Chrome, it seems to me a very useful feature in cases where you don't have access to the backend or you don't want to deploy.