The benefit of having source maps is clear to me: one can for example debug errors with the original, non-compressed source files while running the minified files. The benefit of minimization is also clear: the size of source files is greatly reduced, making it quicker for browsers to download.
So why on Earth I would want to include the source maps into the minified file, given that the maps have size even greater than the minified code itself?
I searched around and the only reason I could see that people inline source maps is for use in development. Inlined source maps should not be used in production.
This could still be done by referencing external source map files of course, but some people seem to prefer inlining for their build process.