Some sites want to ship small bundles to the client by default, sourcemaps enables that + you get to introspect it because it's downloaded only when requested. Literally best of both worlds :)
To elaborate on your comment, if you just ship sourcemaps in production, that means you can ship minified code and track down what _actual_ source that you _aren't_ shipping to users is getting called, is in stack traces, etc.
a lot of people learned to code on the web via viewsource - now we are obfuscating the code